Sen. Barry Goldwater (R-Ariz.), in line to be chairman of the Senate Armed Services Committee, said this week that President Reagan should freeze military spending at last year's level as a contribution toward deficit reduction and should give up on the MX missile.

Goldwater's remarks represent a reversal of fortune for the Pentagon and White House, which on major issues could almost always count on the vote of his predecessor, outgoing Chairman John G. Tower (R-Tex.).

Goldwater said he will support the military from his powerful new position but wants to reduce the size of the Pentagon staff and rein in defense contractors who until now "pretty much wrote their ticket."

The former presidential candidate, 75, known for his bluntness as well as his conservative views, said becoming chairman of the military committee during what he has announced will be his last two years in the Senate represents "the biggest challenge that I have had" there and caps a career that included 37 years of active and reserve military duty. During an interview Tuesday, he sat at a desk bare of papers but displaying a sign reading "Air Force Spoken Here." On the walls hang scores of models of military airplanes that Goldwater has piloted.

Goldwater, despite his sympathy for the military, departed sharply from administration orthodoxy during the interview by saying that the Pentagon can get along without more money and without the MX missile. Defense Secretary Caspar W. Weinberger has said that defense spending should rise from $293 billion in fiscal 1985 to about $334 billion in fiscal 1986.

"They can live with it," Goldwater said. "They won't be happy. Neither will the post office be happy with the same money they got. Neither will my secretary be happy with the same money she got. But you can't keep pumping out money you don't have."

Goldwater said that he expects Congress to kill the MX. The 10-warhead nuclear missile is the centerpiece of the administration's strategic modernization program, but Congress balked at funding it last year. A new vote is scheduled for this spring, but the Arizonan said he hopes Reagan "would not push this thing."

"Just let the Congress vote yes or no, and then forget about it," Goldwater said. "I don't think he can win it, so why get your ass knocked off. You don't go out and pick a fight if you're going to lose."

Goldwater said he has supported the MX until now. If the Air Force had called it "Minuteman IV," he said, making it seem like a modification of existing missiles rather than a new system, it would be built by now. But he said that "my heart has never been in" the MX.

"I'm not one of these freeze-the-nuke nuts," he said. "But I think we have enough, I think they have more than enough, and I don't see any big sense in going ahead building."

Goldwater, who will take over Armed Services because Tower retires this year, will have to give up his chairmanship of the Select Committee on Intelligence. His tenure there left him with no fondness for that panel, however.

"I think the intelligence people should be left alone," he said.

He said he believes that the intelligence oversight committees in the House and Senate should be scrapped or, at the least, merged into one joint committee.

The Senate Armed Services Committee traditionally has been the Pentagon's best friend in the legislature, but some officials believe that its character is changing. Tower's resignation and the death in 1983 of the committee's ranking Democrat, Sen. Henry M. Jackson (D-Wash.), deprived the panel of two of its steadier and more conservative voices.

Goldwater has said he will not seek reelection when his term expires in 1986, making Sen. John W. Warner (R-Va.) the likely chairman if the Republicans retain control of the Senate, since the ranking Republican, Sen. Strom Thurmond (R-S.C.), already chairs the Judiciary Committee. The Democrats on the panel now include a number of Pentagon critics of varying political stripes, including Sens. Sam Nunn (Ga.), Gary Hart (Colo.), Carl M. Levin (Mich.) and Edward M. Kennedy (Mass.).

The panel agreed last year to reexamine the oft-studied issue of Pentagon reorganization. Goldwater said he believes that change is needed in the way the Defense Department operates, but said he does not know what form it should take.

"It's the whole goddam Pentagon," he said. "I don't think it should take 22,000 people to run the whole hierarchy of the military."

While saying he does not necessarily support reform of the Joint Chiefs of Staff -- often cited as too weak to curb costly rivalries among the Army, Navy, Air Force and Marines -- Goldwater said that the joint staff is too large.

"Why it takes so goddam many men to sum up the thinking and discussion of four men who meet maybe three times a week, I don't know," he said. "But there's too many good military men tied up in the Pentagon who ought to get their asses out and go back out in the field."

Goldwater said he strongly supports continued CIA aid to the "contra" rebels in Nicaragua but believes that Congress will not approve such aid. He said he believes that the administration is following a wise course in Central America.

"I think we have to do anything we can," he said. "That's the one place I would not hesitate to send troops."

Goldwater restated his belief that the United States has "lost the last two wars" in Korea and Vietnam because of civilian interference in the military conduct of those wars. Future wars, he said, should be left to military officers who know how to fight.

But he said the military has not done well in managing the procurement of weapons. He said he will approve creation of a new subcommittee on procurement, to be chaired by Sen. Dan Quayle (R-Ind.), and will urge the services to train officers specifically for management functions.

"My principal effort's going to be in the area of getting costs down," he said. "The so-called armaments industry has been through a long, long period of carte blanche where they pretty much wrote their ticket. . . . I think the industry has already gotten the idea that we're going to be a little tough to get along with."

Goldwater acknowledged, in fact, that everyone might find him a little tough to get along with. While naturally disposed toward the military, he said, he also will not shy away from a fight.

"I may not fill the role as diplomatically as John Tower did," he said, smiling. "I'd much rather go to a man and say, 'What in the hell is your problem?' "