Thai Prime Minister Kriangsak Chamanan arrived in Washington yesterday at a time of intense uncertainty for this Southeast Asian nation.

Not only are Vietnamese troops poised on all of Thailand's eastern borders for the first time in history, but China and the United States -- the two countries most critical for the Thais -- have embarked on a new relationship.

In addition, Kriangsak's talks with President Carter come three months before general elections designed to dismantle martial law and broaden the civilian base of his militarybacked government -- a move he hopes will be welcomed by Washington's human rights advocates.

There is little expectation that American B52s and troops, which left Thailand after the Indochina war, will return to nurse the country through the delicate adjustments demanded in coming months, but Thailand's -- and Kriangask's -- relations with the United States remain vital.

He will want arms and advice from Washington to help deal with the new Hanoi-dominated Indochina; a stronger commitment to help resettle the 140,000 Indochinese refugees in Thailand; more aid for narcotics suppression; and political support to help him confront domestic opposition.

[In Washington, U.S. officials, speaking privately, said the United States plans a significant increase in its program of assistance for Indochinese refugees, the Associated Press said.]

Above all, Kriangsak will want to gauge for himself how much room the Chinese and U.S. opposition to Soviet "hegemonism" allows him to deal with the enhanced power of the Soviet Union and Vietnam in Indochina.

As a token of its concern, the United States has already announced it will restore $6 million in foreign military credits, bringing the total to the $30 million promised earlier this year when Vice President Mondale visited Bangkok. Kriangsak will also seek approval to increase foreign military sales to Thailand from $100 million thsi year to $400 million for fiscal 1980.

The Thais want TOW antitank missiles, M60 tanks, heavy artillery, Side-winder air-to-air missiles for two new squadrons of F5E fighter planes, and communications equipment.

Critics here charge that Kriangsak's shopping list appears designed to strengthen his defenses against hardliners in the militry. And they said, in view of Vietnam's military superiority, any armaments program will only distract from urgently needed social reforms.

Many consider diplomacy to be the more realistic response to the new regional power balance. Thailand is important to China's plans to supply th Khmer Rouge forces fighting the Vietnamese Army in Cambodia. Hanoi could retaliate by escalating support for Thai Communist insurgents whose sancturaries are mostly in Vietnamese-controlled areas of neighboring Laos and Cambodia.

Before Kriangsak allows China to derail his policy of strict neutrality in the Sino-Vietnamese conflict, he will want to know the U.S. view of the situation in the region.

The United States is hardly likely to openly endorse Chinese plans for assistance to the Pol Pot forces in Cambodia. But with Teng Hsiao-ping, Prince Norodom Sihanouk and Kriangsak all in Washington within a few days of one another -- and Sihanouk agreeing with Teng to return to Peking to lead the anti-Vietnamese struggle -- Kriangsak will have to be careful to offset the impression in Hanoi and Moscow that Thailand has assumed its place in the Sino-American "antihegemony" bloc.

Kriangsak would also like to get credit in Washington for holding elections, for his antinarcotics drive, and for his humane treatment of Indochinese refugees.

His year in office has led to relaxed political tensions, but he has yet to introduce meaningful reforms. The new constitution virtually assures his continuing as prime minister, appointed by the nonelected members of the legislature he himself appoints. And, before the elections, his martial law government has slipped through a tough anticommunist act which extends the state's already broad and arbitrary powers of search, arrest and detention.