President Reagan's choice for national security adviser, Lt. Gen. Colin L. Powell, will have an unusual Senate confirmation hearing on his military promotion before assuming his full title as national security adviser, White House officials said yesterday.

The officials said Powell consented to the hearing before Senate Armed Services Committee Chairman Sam Nunn (D-Ga.). Nunn is a member of the Iran-contra panel, which recommended in its report last week that the national security adviser should not be an active military officer "as a matter of policy."

The recommendation was voted on the day Reagan named Powell to succeed Frank C. Carlucci. Several committee members said last summer, at the time former national security adviser John M. Poindexter testified, that active duty military officers should not be national security advisers.

The Senate hearing was triggered by Powell's military promotion, according to White House spokesman Marlin Fitzwater.

Powell holds the permanent rank of major general in the Army, but was promoted to lieutenant general when Reagan chose him as deputy national security adviser. Such promotions require Senate approval but Congress waived the option of a hearing at that time, officials said.

Now that Reagan has tapped Powell to be national security adviser, the Senate again can call a hearing on his military promotion. This time, Nunn decided to hold a hearing. Under an agreement with the White House, Powell will serve as acting national security adviser until after the confirmation.

It is highly unusual for a president's choice for national security adviser to appear at such a hearing. Reagan and previous presidents have resisted calls to subject the post to Senate confirmation. Fitzwater, who has defended Powell as a highly qualified military officer who overhauled procedures at the National Security Council in the wake of the Iran-contra affair, said yesterday that Powell is a "special person and a special case" who "deserves a hearing."

Other White House aides said they anticipated that questioning would be limited to Powell's background and qualifications. However, Nunn may want to question Powell on other topics, including his role in the Iran arms sales while serving as military assistant to former defense secretary Caspar W. Weinberger.

No hearing date has been set, White House officials said.