President Reagan's effort to boycott and isolate Beirut International Airport has filled the war-weary Lebanese population with bitterness and jolted its government into improving conditions at that vital facility, one of Lebanon's last symbols of survival.

Although the American move has not changed much in the airport's already sluggish activity, Lebanese officials, with backing from Syria, have given improved security and refurbishing plans top priority in the past few days. Bulldozers are repairing damage and blocking off infiltration routes used by gunmen and plans are being drawn to rebuild the radar system, out of commission for months.

Lebanese ambassadors in European capitals have been lobbying feverishly with their host governments to keep them from joining the boycott that the Reagan administration imposed in what it said was an effort to curb terrorism after hijackers of the Trans World Airlines plane repeatedly used the airport.

Some Eastern European airlines, in an apparent move to win friends in Lebanon and cash in on the business opportunities, already have resumed flights to Beirut, arriving only partially filled but leaving here with full loads of hard-currency passengers willing to fly anyplace in Europe where they can get a reliable connecting flight to their destination.

The periodic shutdown of Beirut International Airport, where 30 foreign airlines once operated regularly and which provides a livelihood for about 20,000 Lebanese families, has jarred the country economically and politically. But it also takes a psychological toll on individual Lebanese, who see the fortunes of the country's only international airport mirroring their own.

"When the airport is closed, I feel I am suffocating, even if I don't want to travel anywhere," said Lamia Suleiman, a secretary. "Just knowing I could leave if I had to is reassuring."

Residents of Beirut, far from complaining about the noise of the traffic from the airport in their southern suburb, have often cheered the deafening sound of planes from their balconies after long airport closures.

In the past, the frequent closings resulted from shelling and hostilities in the immediate area of the airport, which abuts territories used by Shiite Moslems and Druze militiamen, with Palestinian guerrillas nearby and, in the past, Lebanese Christians and Israeli troops as well.

But the most serious threat to the airport now is the boycott that the United States is trying to arrange as a result of the TWA hijackers' use of it. "The purpose," Secretary of State George P. Shultz said last week, "is to place off-limits internationally that airport until the people of Beirut put terrorists off-limits."

A senior government source here expressed "great disappointment with Washington, which is treating the symptoms, not the cause."

Selim Salam, the chairman of Lebanon's national flag carrier, Middle East Airlines (MEA), which has been barred from flights and ticket sales in the United States, put it this way: "It's like beating up my small child instead of fighting with my wife. It's too hard a punch for a small child."

Salam, whose airline has lost $40 million since 1975 because of forced closures that have amounted to 600 days, said, "Every airport in the world has shortcomings -- we have more than others." He called the U.S. boycott a "ruthless penalty for such a small, war-torn country."

"It makes life more miserable, and it will increase our workload and frustration, while we are trying desperately to survive," he said. He noted, however, that it has set a process in motion for security improvements not only at Beirut Airport, but also at others.

Khalid Saab, the airport's deputy director, says it is wrong to expect that Beirut Airport could have been immune to the chaos in its surroundings in the years of warfare and lawlessness. "The airport is not Plato's Republic, it's part of Lebanon," he said, "but its continuation is imperative."

While the gunmen who strutted freely around the airport compound during the TWA hostage crisis have been pulled out, there still remains a lot to be done at an airport that has been in a war zone for 10 years, shelled repeatedly and closed for long stretches, the last of which lasted six months.

Prime Minister Rashid Karami said today that a major effort, "advised and assisted" by Syria, would begin at once to bring security to the airport and the surrounding area. A seven-man committee formed to oversee the new security measures -- including representatives of the police, the Lebanese Army and Shiite and Druze militias -- will hold its first meeting Friday morning, Karami said.

With the various militias' penchant for fighting each other and the Lebanese Army's record of indecision and even partisanship in the face of factional fights, Syria's commitment to put pressure on all sides appears to be the key to secure operation of the airport.

A $1.3 million project to improve airport security and safety was started before the hijacking after the commander of the airport's security agency, Brig. Yassin Sweid, left his job in disgust and offered his resignation over his men's helplessness in making the airport safe in the face of gun-brandishing militias. He has set several conditions -- including a beefed-up security force -- before he will return.

Saab said that illegal access routes to the airport grounds have been sealed off with earthworks and that construction has resumed on a five-mile wall around the complex, reinforced with barbed wire and electronically controlled emergency gates.

Although Saab admitted that revamping the airport and making security foolproof will take some time, he emphasized that "there is a formidable will on the part of all parties to keep gunmen out and keep the airport going."

In prewar days, about 30 international airlines used to call at the airport in Beirut, once the busiest in the region, but now few other than MEA use it.

The Romanian airline Tarom resumed flights Tuesday, after simply announcing that it was returning. East Germany's Interflug, with some prodding from Saab, resumed service today, with a flight that brought 25 passengers to Beirut and flew 150 -- a full load -- out, with more would-be passengers on a waiting list. Balkan Airways and Aeroflot also have indicated a readiness to resume service, Saab said.

During the decade of Lebanon's civil war, MEA's passenger traffic has dropped from 1.1 million in 1974 to 550,000 last year. Customs revenue has dwindled to one-fifth of prewar levels.

MEA is still flying its regular routes to Western European capitals, West Africa and the Persian Gulf, but at reduced frequency. It is still using its 15 Boeing 707s and 720s, but its three 747 jumbo jets have been grounded or leased to other airlines for the time being.

Trans Mediterranean Airways, Lebanon's cargo carrier, which is also affected by the U.S. sanctions, temporarily has rebased its seven 707 freighters in Sharjah in the United Arab Emirates because of the slump in business in Lebanon.

With 5,000 workers on its payroll, MEA is Lebanon's largest private employer, and the cargo line employs another 1,000. Related airport services provide work for approximately 14,000 more Lebanese.

Ossama Chahine, a university professor waiting at the airport yesterday for relatives to arrive from Dubai, said he tried but failed to understand the logic behind the U.S. action.

"It's always easy to punish Lebanon, because it's the weakest of all," he said. "When a country as great as America passes such a verdict on a small nation, it is in fact telling it you have no right to live instead of helping it to survive."

"Americans should appreciate that people here have human value and dignity. All problems have reasons and all ailments have a cure, which is not war," said a Lebanese Armenian at the airport who declined to be named.

Wadad Sfeir, standing nearby, complained that, in her view, the United States gave Israel the green light to invade Lebanon and it vetoed any U.N. Security Council condemnation of Israeli actions in the south, "as if the hijacking of one plane is something and the hijacking of a whole country is something else."

Many ordinary Lebanese feel that they, too, are victims of the kind of international terrorism that led to the TWA hijacking -- and in addition lack an assertive state that could protect them.

In 1976, an MEA plane was blown up on its way to the Persian Gulf, killing 130 persons. The TWA hijacking was the fifth in the region this year, prompting some Lebanese to make the difficult trip by sea to Cyprus to avoid the hazards of crossing sectarian lines in Beirut as well as those at the airport.

A decision was made in Damascus this week, in a meeting of Lebanese Moslem and Druze leaders and Syrian officials, to appoint Syrian observers to a special coordination committee in charge of supervising security at the airport and routes leading to it.

Since Syria has become the strongest power in Lebanon, the move prompted widespread hopes here that it will boost Lebanese government efforts to preserve the safety of travelers and guard the airport from armed incursions.

The Lebanese Army force that is to have a role in airport security is to be a special mixed unit with troops from all brigades and religious sects, a move that is intended to prevent a recurrence of past situations in which, for example, predominantly Shiite Army units joined forces with Shiite militias.

Already, security checks have been stepped up at the airport, with thorough baggage searches. But their efficiency remains in doubt because, with electronic detectors still inoperable, all searches must be done manually.

Airport deputy director Saab complained that it appeared a campaign was under way to further undermine the airport's reputation. Amid the efforts to restore order in the chaos, he said, there had been two hoax bomb threats in the past week that had caused new, but unwarranted, panic.