Sen. Charles Percy said yesterday he will tell Soviet leaders that if they and President-elect Ronald Reagan can negotiate a revised strategic arms limitation treaty (SALT II), it will be approved handily by the incoming Republican-controlled Senate.

The Illinois Republican, who is line to become chairman of the Senate Foreign Relations Committee, will begin a visit to Moscow tomorrow. Although the trip had been scheduled before the Nov. 4 elections, it has taken on a new dimension since the Republicans won control of the Senate.

At a news conference yesterday, Percy said Soviet leaders have been frustrated in recent years because they could not be certain that agreements negotiated with the United States would be approved by Congress. Then, referring to the "mandate" given to Reagan and Senate Republicans, he asserted:

"That mandate assures that if the Reagan administration goes into negotiations and comes to an accord, I can assure them that we can back it solidly in the Senate."

Percy, who said he has discussed his Moscow trip at length with Reagan and his chief foreign policy aide, Richard Allen, said he will underscore to the Soviets that the election results will enable the new administration and the Senate to speak "with a single voice." He added, "They must clearly understand the mood of this country."

The SALT II worked out with the Soviets by President Carter was put on hold before it went to the Senate floor for ratification debate because of the Soviet invasion of Afghanistan last December. Reagan, who has denounced the accord as detrimental to U.S. interests, said in the presidential campaign that he will seek to have it renegotiated.

Despite Reagan's pledge, Percy said he is not sure yet whether Reagan, once he assumes the presidency, will withdraw the treaty from the Senate. That, he said, is a matter to be decided by Reagan and his secretary of state.

But, he added, "I can't imagine you can begin discussions with the Soviet Union and overlook seven years of negotiation."

Elaborating on his contention that the present treaty can be used as a springboard to negotiate a new, more satisfactory accord, Percy noted that, while he voted for SALT II in the Foreign Relations Committee, he considered it "flawed" and had planned to try to amend it on the Senate floor. Amending the original agreement would have required further negotiations with the Soviets to secure their approval for the changes.

In response to questions on other subjects, Percy said he plans to offer legislation that would require any future economic sanctions imposed by the president to be "across the board" rather than limited to a single industry such as agriculture. But he also suggested that Reagan, instead of simply lifting the grain embargo against the Soviet Union, could use it as a bargaining counter in dealing with Moscow.

He said he already has sent word to the Soviets, in his capacity as a senator from a state with a large Polish-American population, that there should be "no miscalculation or misunderstanding" about the danger of interfering in the labor unrest gripping Poland. Percy added that, while he has discussed the Polish situation "at considerable length" with Reagan's advisers, he will not be talking to Soviet leaders about it "as an emissary."