A House Armed Services subcommittee is expected to vote today to cut $1.2 billion from the $3.7 billion requested by President Reagan for his Strategic Defense Initiative (SDI), the so-called "Star Wars" research program, according to congressional sources.

The Senate Armed Services Committee last month removed $300 million from the administration request.

The House reduction was proposed by the subcommittee staff as part of a package of cuts that would reduce the president's overall fiscal 1986 defense spending request by almost $20 billion.

That would provide the Defense Department with $302.5 billion, a so-called "zero real-growth" budget that permits spending next year at this year's level plus an allowance for inflation.

Yesterday, the panel's procurement subcommittee approved funds for 21 new MX intercontinental ballistic missiles, down from the 48 requested by the president and approved by the Senate Armed Services Committee. The Senate committee limited the number of MX missiles that could be based in silos to 50.

The House subcommittee defeated, 10 to 4, an amendment to put a cap on deployed MX missiles at 40 and buy no new ones next year. That proposal, sponsored by Reps. Dave McCurdy (D-Okla.) and Nicholas Mavroules (D-Mass.), will be reoffered before the full committee and later on the House floor.

Congressional sources said they expect it to pass the full House even if it fails in committee.

Overall, the procurement subcommittee cut almost $7 billion from the administration request, removing two types of Navy aircraft -- additional P3 antisubma- rine planes and new Tacamo aircraft providing communications with submarines on patrol underwater.

The House subcommittee reductions were $2.4 billion more than those made by the Senate committee.

While reducing various weapons systems, the House panel voted to require the secretary of defense to assure an Air Force competition between General Dynamics' F16 and Northrop Corp.'s F20 for tactical air fighters to be purchased in the five years starting in fiscal 1987.

Northrop, which began the F20 as a cheaper fighter to be sold to foreign governments, has increased its capabilities and has offered to sell it to the Air Force at a price lower than the F16's. The Air Force had planned to continue purchasing the General Dynamics plane.

On Tuesday, the seapower subcommittee reduced the president's requests by $850 million without cutting back on any ships. The reductions, one member said, came from savings.

Several committee members said yesterday that attempts will be made in the full committee markup to cut SDI even further. One Mavroules amendment is expected to try to push the spending down to $1.4 billion, this fiscal year's level.

One member said he expected the full House to approve $1.8 billion for SDI.

That figure has been proposed by a bipartisan group of senators and recommended by several outside experts who support defense system research but oppose the president's effort to develop a space-based system.