President Reagan proposed a major reduction yesterday in the nation's stockpile of strategic materials for use in wartime, charging that the Carter administration had vastly overestimated the amount that would be needed and dropping plans to purchase materials worth $9.7 billion.

Congress is expected to look critically at the proposal, which would trim the stockpile from $16.3 billion in materials envisioned by the Carter administration to $6.6 billion.

The White House said in a statement that the Carter administration had used "basic errors and unrealistic assumptions" in its 1979 study setting goals for the National Defense Stockpile.

Richard Levine of the National Security Council said the stockpile might grow somewhat if new, high-technology materials planned for weapons systems in the 1990s are needed.

The Reagan plan covers 42 materials, compared with 62 in the Carter plan. A White House official, who refused to be identified, said the other 20 materials, which will be studied later, probably would not add more than $500 million.

Congress created the stockpile to meet "military, industrial and essential civilian needs" for a three-year, conventional global war, the White House said.

The White House official said a number of factors led the administration to conclude that a smaller stockpile would suffice. For example, he said, today's high-technology wartime hardware tends to be less "material-intensive" than earlier versions. He said the Reagan study also used a more accurate model of the Pentagon's requirements.

A White House statement also said the new stockpile did not include materials for producing "nonessential consumer" goods.

Sen. Gordon J. Humphrey (R-N.H.), chairman of the Senate Armed Services subcommittee on preparedness, is planning hearings on the administration proposal and is withholding judgment for now, an aide said.

Congress is expected to question whether the Reagan stockpile is adequate, staff aides said. Two pending amendments, one to the defense authorization bill and the other to the supplemental appropriations bill, would essentially freeze the stockpile at the Carter administration's levels.

The White House official said Reagan would oppose these amendments.

The Carter administration figured that it had $3.5 billion in surplus materials, largely silver and tin, but the Reagan administration estimates the surplus to be $3.2 billion, which includes a wider array of materials, the White House official said.

The president announced in a statement that he would sell $2.5 billion in surplus materials over the next five years, using the proceeds, in part, to reduce the deficit.