The possible sanctions against Lebanon discussed yesterday by the Reagan administration are likely to have little impact on the hijackers holding the 40 American hostages and could make it more difficult for the Lebanese to maintain their independence from Syria, acccording to non-government experts, Lebanese diplomats and some administration officials.

White House officials said yesterday Reagan may try to close the Beirut airport and blockade Lebanon if diplomatic efforts fail to free the American hostages.

"The realistic question is who is hurt by this," said a U.S. official.

"It's not a step in the right direction. It's not going to be helpful," remarked Lebanese Ambassador Abdallah Bouhabib.

Beirut airport is virtually closed down now anyway. The official Lebanese Middle East Airlines is the lone commercial carrier still using the airport.

Squeezing Lebanon economically is even more difficult. There are a dozen or more illegal ports along the Lebanese coast that are used by Lebanese smugglers and merchants and take as much trade as Beirut harbor, if not more.

In addition, Lebanese and U.S. officials predicted that Lebanese merchants, known for their resourcefulness, would simply resort to road routes to Syrian ports to the north, costing them more but not cutting off trade.

The overall effect of the two measures, Lebanese and other U.S. analysts agreed yesterday, would likely be to redirect all Lebanese trade and travel through Syria, thereby even further increasing Syrian control over the entire country -- a long-term Syrian objective.

"It benefits Syria," remarked Fadi Hayek, head of the Lebanese Information Center, the public-information office here of Christian militia Lebanese forces based in East Beirut.

"It will ensure Syrian control," he added.

White House spokesman Larry Speakes indicated yesterday that the administration hopes the prospect of the airport's closure and an economic boycott would stir a "certain number of leadership elements" in Lebanon to apply pressure on the Shiites to free the American hostages.

But the reaction from the Lebanese Embassy, joined by the Lebanese Information Center here, served to underscore the difficult task facing the administration in seeking to find effective nonmilitary ways to increase pressure on the Shiites without undermining its own policy toward that war-fragmented nation and alienating the entire Lebanese population.

"It's not a case of state-supported hijacking," snapped Ambassador Bouhabib. "Taking such measures are against the government of Lebanon, the people of Lebanon, not the hijackers."

Bouhabib said he had already been instructed by his government to protest the measures officially if and when they are officially announced by the White House.

Even some State Department officials expressed doubt that those two measures would have much effect.

Some expressed fear that they might even be counterproductive to the declared American objective of helping to maintain an independent Lebanon.

The closing of the airport, which administration officials indicated they hope to accomplish through an international boycott, began to emerge as the way of eliminating what U.S. officials regard as one of the world's last remaining "safe havens" for hijackers today.

Secretary of State George P. Shultz set forth what now appears to be the official administration rationale for such an action during an appearance on ABC's This Week with David Brinkley last Sunday. Shultz pinpointed Beirut airport as responsible for 15 percent of all the hijackings that have taken place over the last 15 years.

"It's surrounded by a community that is willing to give space to terrorists, so the Beirut International Airport is definitely a problem," he said in an apparent reference to the nearby Shiite suburbs.

Over the past three years, the airport has repeatedly been shut down for months at a time by fighting and threats from one militia or another.

It was closed continuously from Feb. 6 to July 9, 1984, and Lebanese have already become accustomed to using other routes.

Christian Lebanese living in East Beirut, fearful of crossing through Shiite-controlled West Beirut, even today mostly take ships from the port of Junieh north of the capital to Larnaca, Cyprus, or travel to Damascus to catch planes, as do Lebanese Moslems when the airport is closed.

Ironically, the other main users of the airport today are the American broadcast networks, which have been running charter aircraft back and forth from Cyprus on a daily basis.