The White House yesterday backed down from the threat, issued Thursday by Office of Management and Budget Director David A. Stockman, that President Reagan will veto a tax bill that makes cuts in only one year.

Deputy press secretary Larry Speakes said that Stockman, who made the comment in testimony before the House Budget Committee, was expressing his opinion as well as what he would urge Reagan to do if confronted with one-year tax-cut legislation.

While Stockman's recommendation would carry "great weight," Speakes said, the president will not decide whether to sign or veto tax-cut legislation until it reaches his desk.

Reagan is determined to fight for enactment of his tax bill, which would cut marginal rates across the board by 30 percent over three years.

But Democrats in the House, including Ways and Means Committee Chairman Dan Rostenkowski (Ill.), favor a one-year tax-cut bill, with additional cuts in later years handled in separate legislation. Congressional sources say the one-year approach is what Congress is likely to send to the president.

The White House has seemed uncertain about how to handle the veto threat. On the one hand, Senate Republicans have urged the president to issue such a threat to pressure Rostenkowski and others into going along with a three-year tax-cut plan.

On the other hand, it is almost unheard of to threaten a veto this far in advance of enactment of legislation, because it robs the White House of much of its flexibility.

Speakes' contradiction of Stockman yesterday was not the first time this uncertainty of approach has been evident. Earlier, Rep. W. Henson Moore (R-La.) said Reagan had told him he might veto a tax bill that differed substantially from his proposal. But when the president was asked about this, he said it was too early to talk of vetoing the bill.

On another subject, Speakes said Reagan agreed with Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) that the top legislative priority should go to the administration's economic program.

But Speakes refused to be drawn into a discussion of Baker's statement Thursday that Senate Republicans had agreed to postpone consideration of such explosive "social issues" as abortion until next year, saying that was a matter for the Senate to decide.

The White House also announced the nomination of John J. Louis Jr., chairman of Combined Communications Corp., to be ambassador to Great Britain, and said that Frank A. Ursomarso, Reagan's campaign television production coordinator, would become director of the White House office of communications.