Syrian troops manning checkpoints here on New Year's Eve wore party hats and gulped champagne from bottles handed them by partying Lebanese. Some of the armed soldiers were staggering drunk by dawn, according to Lebanese and Westerners stopped at the checkpoints.

The gradual corruption over the past three months of the once-ascetic army first amused Arab and Western observers here. But now Arab diplomats and Palestinians here are concerned that the easy life of Beirut is sapping the Syrian army's ability to fight.

The officers, on whom Syrian President Hafez Assad depends to form the backbone of his regime in Damascus, attend fancy dinner parties here at which they are wined and dined by Lebanese seeking favors from the Arab peacekeeping force.

The Syrian troops, who make up the bulk of that force, copy the officers. Some smoke hashish which is readily available on the streets here. Many frequent tawdry bars where they can get wine, whiskey and women at a price they can afford. Some are seen drunk in the streets coming from these bars.

This is common behavior for most armies, but the Syrian force - considered by some to be the best fighting unit in the Arab world - never before has been subjected to such temptations.

Syria has stationed its crack troops in Lebanon - units of the 3d Armored Division, which usually is held in reserve to repulse an Israeli invasion over the Golan Heights. According to a Western diplomat in Damascus, the division is the first line of Syria's defense.

Now, one Arab diplomat said here, it will take a year of basic drills - including six months in a harsh desert environment - before this army is toughened enough to become an effective fighting force again.

"We're worried about it," said a Palestinian fighter who believes there may be another war with Israel in the next two years. "We may have to fight with them again and I don't like to see them going to hell like this."

Western and Arab observers have noticed the Syrian troops are not keeping their heavy weapons properly maintained. The Soviet-supplied T-54 and T-62 tanks are getting rusty from sitting in the open during Lebanon's rainy winter season. The area where most of the tanks are kept has turned to mud bogs and the tanks are protected only by tarps thrown over them. The heavy guns on these tanks have not been properly greased to protect them from the constant dampness, Western observers said.

"They are in horrible condition. Just look at them yourself," said one observer. "The only guns they keep greased are the machine guns in the tank turret because they think they may have to use them."

The 37mm anticraft guns, positioned at key intersections to be used with deadly accuracy against buildings, are also rusting. The delicate gears that control their direction of fire are encrusted with dried mud. Bare metal shows at places where the guns have been chipped.

Generally, this routine maintenance is enforced by officers who have tight control over their men. But with the Syrians spread so thinly through this country - 30,000 soldiers, but stationed at every important crossroad or government building and along all key highways - small squads of soldiers are left alone without the constant supervision of their officers.

While the Syrian troops billets would be considered spartan by Western standards, they are luxurious compared to the soldiers' usual quarters.

Many of them have taken over abandoned buildings where they have heat and hot water. Their food, usually mixtures of rice and lamb, is brought to them in a mess truck and served generously with Arabic bread.

The soldiers try to be friendly. If they speak English they will often ask American reporters to take their picture. A soldier guarding the Ministry of Information yesterday had placed a rose down the barrel of his machine gun.

The Syrian soldiers, mostly farm youth, have seen things in the traditionally free and easy city of Beirut that they never saw at home.

Cartons of Marlboro and Kent cigarettes are hawked on street corners. In Syria, they rolled their own smokes. Beer, whiskey and wine are easily available. Lebanese women, especially in Beirut, saunter down the streets in provocatively tight jeans and blouses. Soft-porn movies are advertised at many downtown theaters and women stand at the doors of dimly lit bars to lure the soldiers in.

Even if such temptations existed in rigorously Moslem Syria, the soldiers would be unable to afford them on their $50-a-month salary. But they get bonuses of $100 to $150 a month for serving in Lebanon. With food, lodging and clothing provided free, that leaves plenty of cash for pleasure.

Officers, who make $125 a month, get bonuses of $240.

Cigarettes cost about $4 a carton and Scotch recently went up to $5 a bottle. Syrian soldiers, who usually carry their automatic rifles even when off duty, often go to the movies for nothing. Prostitutes charge only about $6, cab drivers here report.

"They have visited Babylon. They have seen Sodom and Gomorrah. How is Assad going to keep them down on the farm now they have seen Beirut?" said a Western diplomat here.

The Syrian troops, who entered Lebanon in June to fight the Palestinians, took on their new role as peacekeepers when an Arab summit meeting arranged a cease-fire in mid-October.

A month later they rolled into Beirut on roads strewn with flowers and grateful Lebanese welcomed them for ending 19 months of bitter civil war.

Now, the welcome is less fervent. While the Lebanese still cater to the Syrians - who are both a police force here and a deterrent against further fighting - many feel the Syrian rule has been too heavy-handed.

Syria closed down newspapers it did not agree with. Moreover, there is a long-standing feeling among many Lebanese - Moslem and Christian - that they are superior to their Syrian neighbors.

So at dinner parties now, Lebanese tell the equivalent of Polish jokes about the Syrian troops.

An example, told at a party the other night with Lebanese, other Arabs and Westerners present: two Syrian soldiers stopped a Volkswagen and opened its rear end to check for arms or stolen goods. One Syrian said to the other, "This man not only stole an engine but it is still running."

On the streets, shopkeepers who consider themselves more sophisticated than the Syrians laugh at the soldiers after they make purchases and call them country bumpkins.

"It's as if someone's come from a farm and is thrown into New York city for the first time," explained one.