President Reagan scored a major victory last night as the House Appropriations Committee reversed its defense subcommittee and voted, 25 to 23, to approve $1.9 billion for the MX land missile he considers vital to his strategic program.

A coalition of nine Democrats, mostly conservatives, and 16 Republicans turned the tide on the MX shortly before the full committee reported its $196.4 billion fiscal 1982 money bill to the floor.

Asked to explain the reversal, Rep. Jack Edwards of Alabama, ranking Republican on the defense subcommittee, replied: "A lot of phone calls." He said Reagan phoned many House committee members, while Defense Secretary Caspar W. Weinberger called others.

Rep. Joseph P. Addabbo (D-N.Y.), chairman of the defense appropriations subcommittee, had prevailed on the MX last month when his unit voted, 7 to 6, to withhold the money until the administration produced a specific plan for basing all 100 of the missiles Reagan intends to buy.

Addabbo and his allies intend to renew their fight against the MX and the B1 bomber when the money bill is debated on the House floor tomorrow. Both sides predicted close fights on those weapons as well as on the reduced total amount approved by the full committee last night.

"There's going to be a mass attack on us by the House Armed Services Committee," Addabbo told his colleagues last night in warning that the committee has mobilized to restore most of the cuts.

The $196.4 billion bill is $4.4 billion less than Reagan had requested and $12 billion less than the Senate Appropriations Committee is expected to put in its money bill today.

Sen. Ted Stevens (R-Alaska), chairman of the Senate defense appropriations subcommittee, urged colleagues during the markup of the Pentagon money bill yesterday to stick with the high figure to gain leverage over the House in negotiations to draft a compromise measure.

The House Appropriations Committee vote eliminated the $35 million the Reagan administration had requested to build a communications system for submarines

The committee refused to go along with a plea from Rep. William V. (Bill) Alexander Jr. (D-Ark.) to change language in the report on the defense appropriations bill to make sure that the McDonnell Douglas C17 aircraft was not frozen out of competition for the role of transporting military gear to the Persian Gulf and other distant spots.

Alexander complained that the committee report all but rules out the C17, chosen by the Air Force after a lengthy selection process. The advantage is going instead, Alexander said, to a new version of the Lockheed C5 transport.

To give the contract for this new transport aircraft, called the CX, to the Lockheed entry "would be a disaster," Alexander said. He complained that the Pentagon is restraining the Air Force from telling Congress that it prefers the C17 to the C5.

Democrat Bo Ginn, representing Georgia where the C5 is made, countered that "we could save billions of dollars" by buying a rework of the existing C5 rather than developing and producing a new airplane such as the C17.

The battle of who should build the CX is expected to resume today when the Senate Appropriations Committee marks up its defense bill. Democratic Sen. Thomas F. Eagleton, representing McDonnell Douglas' home state of Missouri, is expected to insist that language in the Senate report be broadened to make clear to the Pentagon that the C17 should remain a candidate for the CX role.

Sen. Mark O. Hatfield (R-Ore.), chairman of the Senate Appropriations Committee, urged his colleagues to send the defense money bill to the Senate floor today. The strategy is to add the bill to the continuing resolution expected to be voted on tomorrow. A House-Senate conference would follow.

"I have no clear signal from the White House about the possibility that the White House will veto the continuing resolution," Hatfield said. He added that he would try to get the compromise defense bill to the president by Thursday.

In an attempt to dissuade Reagan from vetoing the continuing resolution that would contain money for the Pentagon and other government departments, Sens. Mack Mattingly (R-Ga.) and James A. McClure (R-Idaho) were working on an amendment that would empower Reagan to make limited across-the-board cuts in government programs, including entitlements such as Social Security.