Faced with Thai appeals to help curtail an "endless" flow of Indochinese refugees into the country, the U.S. Embassy here has announced a new American refugee policy aimed at discouraging Indochinese from leaving their homelands.

After April 30, refugees from Vietnam, Cambodia or Laos arriving in Thailand or other Southeast Asian countries "will be considered for resettlement in America only if they can demonstrate close links with the United States," the embassy said in a press release.

The tougher new policy could eliminate 30 percent to 40 percent of the U.S. refugee caseload, the approximate percentage of those who were admitted to the United States in the past but would not meet the new criteria, a U.S. diplomat said.

The embassy stressed, however, that the policy applies only to refugees arriving in Thailand or other Southeast Asian countries after April 30 and that those already in camps or arriving before then will be processed as before.

Since 1975, the embassy said, 68 percent of the 388,400 Indochinese refugees who have left Thai camps for resettlement abroad have gone to the United States.

The embassy said the new criteria mean that "in the future refugees will only be eligible for resettlement in the United States if they have close relatives in the United States, have worked at one time for the U.S. government or have other ties to the United States or to one of the previous noncommunist governments of Indochina and now have reason to fear persecution."

In Washington, H. B. Cushing, director of the Office for Refugee Admissions at the State Department, confirmed the change in admissions policy.

Commenting that the refugee problem in Southeast Asia is "much reduced from what it was three years, two years and even six months ago," and that the United States has admitted a total of Indochinese refugees roughly equal to the number of those accepted by all other countries together since 1975, he said, "It is not within the capacity of the United States to take everyone who might wish to leave Indochina."

Indicating that refugees without ties to the United States might be taken by some other country, he said the policy change has been made "in order to have a program adequate to take those with ties" to America. "We believe," he added, "it is quite possible that those ties may be some or all of the reason why life for them in Indochina is intolerable and wish to honor that association with us by admitting them."

The new rules are in line with Thailand's policy of "humane deterrence," under which authorities last year declared Laotian and Vietnamese refugees arriving in Thailand ineligible for resettlement in third countries and placed them in "austere" camps until they are ready to return home.

According to U.S. figures, the policy seems to be having the desired effect. So far in April about 700 Indochinese refugees have arrived in Thailand, compared to about 7,600 in April 1981.

Besides the Thai policy, a U.S. diplomat said, other reasons for this could be signals from Washington of restrictions on refugee intake and a shortage of boats in Vietnam.

Because of the Thai policy, many Vietnamese refugees now deliberately sail for neighboring Malaysia. However, the embassy said it believes that the new U.S. rules will also reduce the number of refugees seeking asylum there and in Indonesia, the Philippines, Singapore and Hong Kong.

The policy was greeted with approval by Thai authorities, although it is still unclear what will become of those refugees who arrive and are ruled ineligible for resettlement.

The head of the Thai National Security Council, Squadron Leader Prasong Soonsiri, praised the new U.S. policy He indicated to reporters that Thai authorities had pressed for such measures during a recent visit here by Richard Vine, director of the State Department's Bureau for Refugee Programs. Prasong said Washington was urged to eliminate "pull factors" that attract refugees.

The new policy apparently signals U.S. willingness now to draw a stricter distinction between political and economic refugees. A U.S. diplomat said it would effectively exclude from consideration for resettlement such persons as farmers, fishermen and unemployed young people who seek a better life but have no ties with the United States.