Thailand fears that renewed Communist insurgency in its northern provinces will be the greatest danger resulting from Vietnam's military sweep of Cambodia.

Some Thai leaders are said to fear an eventual armed invasion by the Vietnamese, but most believe the greater threat will be Vietnamese aid to Communists who have been active in Thailand for 14 years, according to well-in-formed source here.

The installation of a Hanoi-oriented government in Cambodia has quickly altered Thailand's outlook on the Communist threat from Indochina.

A few weeks ago, the radical but anti-Vietnamese government of Pol Pot was a buffer between this antiCommunist country and its historic enemy, Vietnam. Now the buffer has collapsed, Vietnamese troops are at the border, and Thailand faces three allied Communist countries led by Hanoi.

With two of them, Laos and Cambodia, Thailand shares a long meandering border that is impossible to patrol effectively and which Thais believe will become an avenue for Vietnamese subversion.

"The Thais had thought they would have a respite because of the quarrels among the Communist nations," said one analyst here. "Now they are very uneasy. Some of them expect an invasion eventually, but the immediate fear is that the Vietnamese will send help and supplies across the border to the Communist Party of Thailand."

The Thai government has said little publicly, except to caution people not to panic and to assure them that Vietnamese troops in western Cambodia have stalled their drives just short of the frontier. In only one case have they driven so close that Cambodian troops were pushed into Thailand.

Nevertheless in Tokyo yesterday, Thai Prime Minister Kriangsak Chamanand reportedly told Japanese officials that his country is the next "target" of Vietnamese aggression and said his forces were on a "full alert," something he had not acknowledged at home.

According to Japanese officials, Kriangsak informed them that he considers the conflict a confrontation ultimately between the Soviet Union, which backs Vietnam, and China, which supported the fallen Cambodian government. He reportedly told them he has "great concern" for the future.

A Thai source here who is familiar with attitudes of top government leaders gave an extremely gloomy projection of the future.

"Vietnam cannot run over Thailand as it did Cambodia," he said. "It will use other methods at which it is highly adept. It will use the ploy of a liberation movement and it will try to foment internal troubles. Invasion is not very likely, but Vietnam will wait until it thinks the fruit is ripe."

It is likely, the Thais believe, that through both Cambodia and Laos the Vietnamese will step up military training, lend psychological support, and provide supplies and medicine to cadres of the Communist Party of Thailand hiding in remote areas.

The guerrillas operate in three distinct areas of northern Thailand and in one southern sector near the Malaysian border. There have been many government-guerrilla clashes lately in the south, but it is with the three northern areas that the Thai government has been most concerned.

Thai officials refuse to discuss the size and disposition of guerrilla forces. Western sources estimate the number of armed guerrillas who operate full time at between 10,000 and 12,000 throughout the country. About a third of them are in northeastern Thailand where the insurgency has been under way for several years.

The guerrillas are supported by militia groups numbering between 10,000 and 15,000, whose main role is to protect villages where the Communists have attained some degree of influence.

The Western sources also estimate that roughly 60,000 to 70,000 other persons sympathize with the insurgents.

Western sources believe that the insurgents' numbers grow at the rate of about 6 to 8 percent a year, but there have been no signs of large-scale movements into the insurgent camp in recent years.

After the October, 1976, military coup that brought Kriangsak to power, however, many student leaders, intellectuals and eight former members of parliament went into the jungles to join the insurgents.

The insurgents emphasize political education and training and attempt to persuade villagers that the Thai government is corrupt and not worth supporting. They operate what one source describes as an "extremely sophisticated" propaganda radio station out of southern China. After winning some political converts, they employ military action to protect their influence and engage in countless clashes with police or border patrols, according to Western sources.

There are also kidnapings, assassinations and continued threats to villages in remote areas. A common tactic, the sources say, is for insurgents to blackmail village headmen, promising to leave them alone in exchange for regular supplies of rice each year.

So far, observers say, there does not seem to be great Vietnamese influence in the Thai Communist Party, although Vietnamese provide supplies and training for Thais who have come into Laos and give them weapons for use by insurgent forces in the northeastern sector.

The Thais are also described as worried about 30,000 Vietnamese civilians who long ago came into Thailand from Laos and who have been denied Thai citizenship. One source, who is skeptical about such concerns, said the Thais look upon the Vietnamese as a potential fifth column.

The existence of a new pro-Vietnam government in Cambodia has raised questions about renewed subversion along the Thai-Cambodian border. The newly installed government in Phnom Penh has said it wants to live in peace with its neighbors, but Thais are dubious.

Vietnamese forces may be in Cambodia for many months, observers here now believe, and the Thais fear they will use that time to prepare subversive actions along the border.