Secretary of State George P. Shultz today opposed the supply of U.S. military aid to the anti-Vietnamese guerrillas in Cambodia, suggesting that lack of constancy in Congress would make such a bold action risky.

Shultz spoke out at a news conference after the House voted yesterday in favor of $5 million yearly in overt military or economic aid to noncommunist factions of the Cambodian resistance.

Aid to the Cambodian guerrillas is likely to figure in Shultz's meetings here Thursday and Friday with foreign ministers of the Association of Southeast Asian Nations (ASEAN), the main group of noncommunist countries in this area.

ASEAN foreign ministers issued an appeal in February for increased international support for the "political and military struggle" of the Cambodian resistance.

Since leaving Washington late last week, Shultz has carefully refrained from any statement of support for U.S. military aid to the guerrillas. He has made positive remarks about possible humanitarian assistance.

Today, Shultz tackled the issue directly, saying that "we don't provide military aid to the rebels and we don't see any special need for it from the United States."

Providing food, clothing, medicine and other supplies, which "people need as much as they need guns," is a better role for the United States "and in the long run, more sustainable," Shultz said.

"Congress is a very changeable operation," Shultz explained under questioning. "They're in favor of something at one time and then some things can happen and they change their mind," he said.

A senior aide to Shultz, apparently referring at least in part to the experiences of Vietnam, said, "We have an unhappy history of having a lot of people who have come to depend on a certain level of U.S. support be abandoned later."

During a visit to a Cambodian evacuation camp in Thailand yesterday, Shultz heard a fervent appeal for direct and open U.S. aid. Residents of the camp displayed signs in English asking for weapons as well as other forms of assistance.

While rejecting military aid, Shultz indicated he will respond to pleas from Congress, the Cambodian refugees and Southeast Asian nations by recommending the supply of U.S. economic aid to the noncommunist guerrilla groups.

Aides to Shultz said the Agency for International Development recently conducted a study in camps along the Thai-Cambodian border. The study recommended training for administrative, educational and medical skills under U.S. sponsorship.

The report is being summarized for presentation to Shultz, amid indications that many of its recommendations will be followed.

Shultz seemed to deny at his news conference that the United States, while refusing to supply military aid to the resistance, has been asking other nations to explore or sustain military aid to the rebels.

An aide to Shultz told reporters later that the U.S. diplomats have appealed to China to support noncommunist Cambodian groups with military aid, partly to balance the arms aid that long has flowed from Peking to the communist Khmer Rouge headed by Pol Pot.

A western diplomat said about two-thirds of the arms aid to the noncommunist factions appears to come from Peking, along with more extensive aid to the communist fighters.

The diplomat, who has expert knowledge of such things, said China is estimated to spend $60 million to $100 million yearly in aid of all sorts to all factions of the anti-Vietnamese resistance.

If these estimates are accurate, U.S. aid of $5 million demanded by Congress would have only marginal impact. In addition to this potential fund, the Reagan administration is reported to be supplying several million dollars yearly in secret CIA funds for nonmilitary purposes to the Cambodian rebels.

In his news conference here, Shultz suggested he may be more flexible than he has sounded during the past several days in connection with an ASEAN proposal for indirect diplomatic talks between the Cambodian insurgents and a Vietnamese delegation that could include representatives of the Vietnam-backed Cambodian regime of Heng Samrin.

In earlier remarks, Shultz stressed the dangers of any diplomatic plan that could seem to give legitimacy to the Heng Samrin government. Today he did not repeat this criticism and said that his "general disposition" has been to support positions taken by ASEAN.

An aide said Shultz has not fully decided how to respond to the ASEAN diplomatic initiative. But the official said it would take a "very powerful reservation" about ASEAN's plan for the United States to turn it down.