Philippine President Corazon Aquino is expected to pay an official visit to Washington in mid-September, two months earlier than first proposed by President Reagan, to speak to a joint session of Congress and to encourage speedy U.S. private investment as the Pacific nation seeks to recover from years of economic decline.

Administration officials said a State Department recommendation that the Aquino visit be advanced to September from November has been accepted in principle at the White House. It is likely to be an "official visit," one step down in protocol from a "state visit," because of White House scheduling requirements, an official said.

Philippine Ambassador Emmanuel Pelaez said Aquino is likely to agree to a mid-September visit when final details of the invitation are worked out.

Rep. Stephen J. Solarz (D-N.Y.), chairman of the Foreign Affairs subcommittee on Asia and the Pacific, was among those who asked that the trip be advanced. He said he is pleased that Aquino apparently will have an opportunity "to present her case to Congress and the country" in September.

Solarz, who delivered a written invitation to Aquino to speak to a joint session shortly after she attained power in late February, said the original plan for a November visit would have brought her to Washington when Congress is likely to be out of session.

"Congress was very intimately involved with the restoration of democracy in the Philippines, and it is entirely appropriate that Aquino address a joint session," Solarz said.

Lawmakers are expected to give final approval in the next few days to the Reagan administration's $150 million special aid package to the Aquino government for fiscal 1986. Solarz said he is seriously considering leading a fight for $250 million more in the current fiscal year, despite the strictures of the Gramm-Rudman-Hollings balanced-budget law, on grounds that the U.S. stake in Aquino's success is high.

Sen. Richard G. Lugar (R-Ind.), Foreign Relations Committee chairman and leader of the congressional delegation that monitored the January election in the Philippines, has proposed an additional $100 million in aid in fiscal 1987.

Secretary of State George P. Shultz, declaring that "I am bullish on the Philippines," urged U.S. business in a June 4 address to "get off the dime and look aggressively at investment opportunities" there. To some extent, Shultz's unusual public advocacy of private investment is intended to compensate for the administration's inability to provide larger sums of official assistance because of U.S. budget restrictions.

Shultz is expected to explore the Philippine economy and possibly to conclude the arrangements for Aquino's trip when he visits Manila June 24-28 for a meeting with foreign ministers of the Association of Southeast Asian Nations (ASEAN).

An Aquino visit here in mid-September could follow important economic and political developments. By mid-August, the Philippines are expected to conclude new arrangements with the International Monetary Fund, a step toward expanded lending by most commercial banks. And a constitutional convention appointed by Aquino is expected to report by early September, starting the Philippines on the road to a new national charter for democratic rule.