The Reagan administration has proposed substantial changes in the nature and amounts of U.S. aid to the Philippines that officials said are aimed at encouraging military and economic reforms by the government of President Ferdinand Marcos.

The Philippine aid portion of President Reagan's budget, amounting to $280 million in fiscal year 1986, was worked out in several months of intense executive-branch deliberations. It is likely to encounter unusually careful scrutiny by congressional committees concerned about instability in the Philippines.

Rep. Stephen J. Solarz (D-N.Y.), chairman of the House Foreign Affairs Asia subcommittee, said it plans hearings beginning late this month on Philippine aid. In view of the "widespread and deep concern" about the situation in Manila, Solarz said, the subject is "clearly the most important issue" to be considered by Congress in the Asian area of the foreign assistance budget.

The subcommmittee staff director, Herbert Levin, is leaving for Manila on a fact-finding trip in connection with the budget request late this week, Solarz said.

Under a 1983 agreement, the administration pledged its best efforts to obtain $180 million per year for five years in return for continued use of two major military bases, Clark Air Base and Subic Bay Naval Base. The sums were supposed to be split equally between military and economic aid.

Last year Congress, led by Solarz's subcommittee, sharply reduced U.S. military aid for the Philippines while making corresponding increases in U.S. economic assistance.

The proposal sent to Congress this week calls for $100 million in military aid, including $50 million in military grants, plus $95 million in economic aid. The total of $195 million is $15 million more than was pledged for the annual bases "rent."

A State Department official said the increase in military aid is to make up for the reductions last year and as "a gesture of support for tentative moves" by the Marcos government toward reform of its military establishment.

Another part of the plan is a new U.S. food aid program of $90 million over two years subject to strict conditions to be negotiated with the Philippines on agricultural credit, price and competition policies. Still another part of the program, unchanged from the past, is $40 million in U.S. development assistance.

The anticipated close congressional scrutiny may intensify the pressures on the Marcos government to improve its political, economic and military policies, an official said.

All this will take place in a situation of uncertainty about Marcos' health. A U.S. official familiar with the situation said Marcos "is not a well man" but declined to estimate how long Marcos could live or remain in power.