The Pentagon's spending spree has backfired among some key congressional conservatives for the first time since President Reagan's election.

Their concern is not as much over the record amounts of money earmarked for defense as it is whether taxpayers will get their money's worth as Reagan accelerates military programs and slows or stops domestic ones.

Is the Pentagon buying weapons too complicated to work in the mud of battle? Is the will of Congress being ignored in the way the Army, Navy, Air Force and Marine Corps plan to buy weapons over the next five years?

Those are two of the questions the House Appropriations Committee has asked a special investigation unit to answer, perhaps in anticipation that lawmakers soon will be charging that the Pentagon is getting more money than it can spend wisely at the same time that domestic programs are being axed or trimmed by the new administration.

The committee chairman, Jamie L. Whitten (D-Miss.), and its ranking Republican, Silvio O. Conte (Mass.), ordered the investigation after the move was requested by Reps. Joseph P. Addabbo (D-N.Y.) and Jack Edwards (R-Ala.), chairman and ranking minority member, respectively, of the defense subcommittee.

The committee's surveys and investigation staff, composed of about 20 specialists from the FBI, the General Accounting Office and other government departments, will conduct the probe.

The Addabbo-Edwards letter requested that a number of major weapons be analyzed to determine whether they have lived up to the cost and performance promises made when their contracts were signed. The investigations unit also is being asked to determine whether Pentagon research chiefs as ordering overly complicated weapons that are difficult to operate and costly to maintain.

The probers also will look into how the Pentagon awards contracts, handles spare parts and controls costs.