The government could buy the same level of national security as could the Reagan administration's five-year $1.55 trillion defense program for $155 billion less, according to the Brookings Institution's annual study of the federal budget. Portions of the study were released yesterday.

William W. Kaufman, an MIT professor who was an adviser to secretaries of defense in every administration from John F. Kennedy to Jimmy Carter and who wrote the defense chapter, said that the Reagan program is "not realistic" and questioned whether it is "adequately related to the dangers that may lie ahead for the United States and its allies."

But Kaufman also raised questions about his own conclusions. He said that the administration probably has underestimated the cost of its planned defense procurement by about $28 billion over the next five years, and has underestimated the cost of operating and maintaining the complex new weapons systems by a far larger amount.

To maintain the new weapons in a state of "high readiness" through 1988 could require as much as an additional $230 billion, Kaufman said.

The annual volume, called "Setting National Priorities: The 1984 Budget," urged Congress to enact some combination of defense and non-defense spending cuts and tax increases to reduce future budget deficits.

Economist Joseph A. Pechman, editor of the book, warned that if the deficits are not lowered once the current economic recovery is well under way, a renewed clash between stimulative fiscal and restrictive monetary policy could mean more years of high interest rates and economic stagnation.

"This is the major economic problem we face," Pechman said. "Unless Congress does something about the budget this year, we are not apt to have a satisfactory economic performance for some years to come."

The book said, "If the Federal Reserve Board were to increase the money supply enough to hold down interest rates, the deficits would generate inflation. If, as seems more likely, the Federal Reserve refused to accommodate that much fiscal stimulus, interest rates would rise sharply and economic recovery would be retarded."

In the longer run, the failure to reduce deficits will mean less investment by business in new plants and equipment and less by individuals in housing because the government will be using a large portion of total national savings to finance the deficits, the book said.

In the defense chapter, Kaufman proposed major modifications of the Reagan program, which he termed the "hot-rod" version of the Carter defense plan. The suggested changes, which would save $223 billion in new obligational authority and $155 billion in outlays between 1984 and 1988, include:

* Scrap the MX missile but leave research and development funds available for a new missile.

* Cancel several aircraft programs, including the B1 bomber, the F14 and F15 fighters, the C5B transport, and the AV8B air support aircraft.

* Cancel construction of 32 planned Navy ships, including three aircraft carriers, nine cruisers, 11 nuclear attack submarines and nine guided missile destroyers, and renovation of two battleships.

* Reduce several Army programs, including the M1 tank, the M2 armored fighting vehicle, the AH64 attack helicopter, as well as the Patriot air defense program, the binary chemical munitions program, and a truck purchase program.

* Reduce programs to stockpile conventional munitions.

* Freeze military manpower at 1983 levels.

Kaufman criticized as not realistic and unnecessarily costly the administration's attempt to create the ability to simultaneously fight a protracted nuclear war, to escalate a conventional attack in one part of the world by another country by attacking that country or one of its allies in another part of the world--called horizontal escalation, and to fight a conventional conflict of indefinite length.