The chairman and the ranking minority member of the House subcommittee on Asian and Pacific affairs asked President Reagan yesterday to review an administration determination that most of the remaining Cambodians in Thailand's main refugee camp are either security risks or "not refugees."

The determination is expected to end processing of Cambodian refugees in Thailand for resettlement in the United States by early next month. In a letter to Reagan, made public yesterday, Reps. Stephen J. Solarz (D-N.Y.) and Jim Leach (R-Iowa) expressed concern that the United States had rejected about 15,000 of the 25,000 Cambodians at Khao-i-Dang, the main refugee camp in Thailand.

Most of the refugees in the camp fled Cambodia after the Vietnamese-backed government took power in 1979. But State Department officials have said all but 2,000 are ineligible for resettlement because they failed to meet the criteria for admission, or had a connection with the Khmer Rouge, the communists accused of killing nearly 2 million Cambodians from 1975 to 1979.

The remaining 2,000 refugees are expected to be processed by June 7, the State Department said.

A subcommittee spokesman said the letter to Reagan was triggered by letters from Cambodians, many of them U.S. citizens, who became concerned about relatives in the refugee camps and in evacuation sites along the Thai-Cambodian border after reports of the end of the U.S. processing. Some refugee officials are concerned that the policy will mean an end to the resettlement of Cambodian refugees in this country. State Department statements that the United States will continue to consider those Cambodians whom Thailand deems eligible as refugees have not dampened that concern.

Solarz and Leach called the screening program "inconsistent" with a 1982 national security directive that says the benefit of the doubt should be given to refugees, especially those who may face persecution if they return to Cambodia.

The subcommittee is expected to hold hearings on the issue in June.

The topic is also likely to be discussed when Secretary of State George P. Shultz meets with Thai Foreign Minister Siddhi Savetsila on Friday, and when Shultz travels to Thailand in July.

The two congressmen also expressed concern for another group of approximately 230,000 Cambodians driven into Thailand by Vietnamese military action late last year and early this year. The Cambodians have been living in evacuation sites along the Thai-Cambodian border. About 2 percent of these people have close relatives in the United States, but because the United States does not recognize any of them as refugees, they are not eligible for resettlement, the congressmen said.

"The way things are right now, if Dith Pran arrived on the border, he would be told, 'Stay on the border,' " said one State Department official, referring to the Cambodian journalist whose escape from the Khmer Rouge is the subject of the movie, "The Killing Fields."

The congressmen are asking that the United States set up a special program to reunite those on the border with their family members in the United States and set up direct educational assistance for the approximately 70,000 Cambodian children in the border camps.