Thirteen Senate conservatives, including presidential confidant Paul Laxalt (R-Nev.), have asked President Reagan to abandon compliance with the unratified SALT II treaty and other strategic arms pacts as long as they are being violated by the Soviet Union.

Their request in a letter written Tuesday came to light yesterday, only a day after the Senate voted, 90 to 5, with support of some of the letter's signers, to urge Reagan to continue U.S. adherence to SALT II provisions with some modifications.

In addition to Laxalt, the signers were Republicans Jesse Helms (N.C.), Malcolm Wallop (Wyo.), Steve Symms (Idaho), Jake Garn (Utah), Phil Gramm (Tex.), Orrin G. Hatch (Utah), Strom Thurmond (S.C.), Chic Hecht (Nev.), Gordon J. Humphrey (N.H.), James Abdnor (S.D.) and Jeremiah Denton (Ala.) and Democrat Ernest F. Hollings (S.C.).

Garn, Gramm, Hatch, Thurmond, Abdnor and Hollings voted Wednesday for the compromise SALT compliance language attached to the fiscal 1986 defense authorization bill. Gramm and Hollings said they voted for it largely because it gives the president more flexibility than did an earlier version to deal with Soviet violations.

The letter noted that Reagan, while refusing to submit SALT II for Senate ratification, has "acted as if the U.S. Senate has ratified it," even while complaining that the Soviets have violated all such pacts.

"Adherence to an unratified, expiring, violated treaty is publicly confusing, constitutionally problematic and strategically unwise. We urge you to end it forthrightly," the 13 senators wrote.

Contending that the Soviets have been violating the agreement while the United States abided by it, they added, "Unilateral compliance is unilateral disarmament."