When communist insurgents recruited Pra Maw La in 1970 and gave him the code name of Comrade Lau Lau, they promised social progress and a better life for him and his followers of the impoverished Karen hill tribe in the remote, jungle-covered mountains of northwestern Thailand.

For the next 12 years Pra Maw La and his clansmen often moved from place to place in this rugged area near the Burmese border. Finally, last month, he decided he had had enough. He led about 300 followers out of the jungle and surrendered to Thai authorities.

"The CPT Communist Party of Thailand cheated us," he said. "They didn't deliver what they promised. They promised to change our society and improve our standard of living. But nothing changed in 12 years."

Today, according to Thai military officials and western diplomats, the Communist Party of Thailand is in trouble, its ranks depleted and demoralized. A rash of mass surrenders and defections of individual party leaders has shown the party to be racked by internal dissension and seriously hurt by curtailment of Chinese aid, the sources say.

Whereas a decade ago a growing communist insurgency threatened to take this country of 48 million people the way of Vietnam, Cambodia and Laos, Thailand today is proving to be the exception by gradually winning its battle for the allegiance of its unassimilated rural inhabitants.

The Thai experience contradicts the "domino theory," in which it was argued that U.S. allies in Southeast Asia would soon fall to communism one by one if the Vietnamese communists succeeded in South Vietnam.

Not only have the examples of its Indochinese neighbors' trials under communist rule hurt the Thai party's ability to recruit, but the Thai government has been winning converts with a liberal amnesty program for members of the outlawed party who come in from the cold.

In addition, the confidence of the Thai military has been bolstered by withstanding the presence--and occasional incursion--of Vietnamese troops along Thailand's eastern border for the last four years following Hanoi's invasion of Cambodia.

"The Thai attitude toward communism has undergone a sea change over the past few years," said one western diplomat. "The Thais no longer have a fear of being a domino quivering and waiting to fall."

The Thai Army's assistant chief of staff for operations, Lt. Gen Prayun Bunnag, expressed this confidence recently when he listed government successes in its counterinsurgency operations and declared victory over the Thai Communist Party. "We can say now that the CPT is no longer in a position to topple the government by force," he told reporters.

The Army's commander in chief, Gen. Arthit Kamlang-ek, has been more cautious. He told an internal security seminar in Bangkok last week that although the party had lost many adherents, "We cannot afford to lower our guard because its leaders still exist and the fight will continue." He said he thought the insurgents might retaliate by turning to urban terrorism.

Founded 40 years ago last month, the Communist Party of Thailand soon fell under the influence of Peking and adopted the Maoist strategy of building a mass base in rural areas. In 1965 the party launched an armed rebellion, clashing with government forces for the first time in northeastern Thailand. As the insurgency spread, the party formed the Liberation Army of Thailand in 1969.

The party's membership received a boost in 1976 when about 3,000 student activists and leftist politicians took to the jungles after rightist elements cracked down on Thai leftists in a bloody confrontation at Bangkok's Thammasat University.

At the peak of the party's strength in the late 1970s, it counted 10,000 to 12,000 active fighters, according to Thai authorities and western diplomats. Then in 1979 China cut back its support as it started to court Southeast Asia's noncommunist nations, which share Peking's opposition to Vietnam and the invasion of Cambodia.

The following year the Thai government adopted the first of its amnesty programs, stressing political rather than military means of combating the communist insurgents. Student radicals began coming out of the jungles in greater numbers along with rural people recruited by the insurgents.

The party now is estimated to number 4,000 to 5,000 hard-core guerrillas. Its greatest strength is believed to be in the south, where government forces must also contend with Moslem rebels, bandits and insurgents of the Communist Party of Malaya based in neighboring Malaysia.

Even so, the Thai Army has scored successes in the region. In February last year government forces overran Camp 508, an elaborate jungle complex that included bunkers and tunnels and was the communists' largest base in the south.

The party fell into such disarray that its fourth party congress could not be held at a single time and place last year but became a fractured series of meetings at three locations between March and May. The result was further internal discord between pro-Peking and pro-Moscow factions and inability to agree on a single secretary general, according to defectors.

In September the party suffered another serious blow when a senior leader, 62-year-old Politburo member Udom Srisuwan, gave himself up along with a dozen relatives and followers. He cited ideological conflicts that had reached "the point of no return" and said the pro-China wing led by elderly communists continued to dominate the party despite the Chinese aid cutoff.

In November, two former members of Parliament who joined the insurgents more than six years ago came out of the jungle separately and turned themselves in. They were followed this month by senior party member Mongkol na Nakhon, 71, who flew in from exile in China.

Mongkol reportedly told friends in Bangkok last week that the Thai Communist Party could never succeed through armed struggle unless the country were governed by a dictator and that about 100 other Thai exiles in China were planning to return soon.

Equally serious for the party's future have been the recent mass surrenders of adherents in different parts of the country.

On Dec. 1, about 250 armed guerrillas turned in their weapons in a ceremony at Mukdahan in northeastern Thailand. They were joined by 750 unarmed relatives and sympathizers.

Then, on Dec. 27, about 200 armed Karen and Hmong hill tribesmen and 400 dependents went over to the government side at a ceremony here presided over by Army commander Gen. Arthit. Thai authorities said the defectors represented 5,200 villagers in the area who had also renounced the CPT but had not attended the ceremony because of the difficulty in reaching Umphang--in some cases a four- or five-day walk from their homes.

The defectors were paid up to $50 for their weapons--mostly old U.S.-made carbines but including some grenades and automatic rifles. They were also treated to a two-day fete of food and entertainment, featuring motorcycle jumping and an American war movie set in Vietnam.

One of the defectors, Lao Ying, told reporters that the party had sent him to Vietnam for a three-month military training course, to China for studies of communist doctrine and to Laos for guerrilla warfare training. He and other defectors cited their hard life in the jungles, with inadequate provisions and no medicine, as one reason for their surrender.

About 70 miles south of Umphang, at a remote site reached by flying in a Thai Army helicopter between jagged, jungle-covered peaks, Lao Ying's former base, called Camp 401, now lies abandoned and partly burned. Invisible from the air under triple-canopy jungle, the bamboo huts of the camp are littered with pages from tracts and remnants of destroyed equipment.

According to Thai authorities, some of the hard-core Communist Party members who lived there and at Camp 302, about 45 miles to the northwest, fled to the south a few months ago while villagers formerly under their control were negotiating a surrender with government officials.