THIS EXTRAORDINARY Congress has now accomplished another feat. It has written a budget -- tight, coherent and respectable -- and it has done that with precious little help from the White House. The budget proposal that President Reagan offered in February had an inexcusable amount of hokum in it and got the deficit down to the congressional target only by grossly underestimating defense spending. Since then Mr. Reagan's attitude toward the emerging congressional budget has fluctuated between indifference and active hostility. His interest has been limited to opposing tax increases and supporting defense increases. He has prevailed on the first point but not on the second. It is hard to think of a year, other than the one in which Richard Nixon was being forced out of office, in which a president took so passive a part in writing the budget. This year, once again, Mr. Reagan refused to deal with the deficit. In his absence, so to speak, Congress has enacted a budget resolution that for the first time in this decade would produce a deficit that convincingly declines.

Defense is the key. Congress has confirmed its decision last year to level off defense spending. Under this resolution the defense appropriations will go up at just about the rate of inflation, or perhaps a hairsbreadth less. Because of the backlog of appropriations, actual spending will continue to rise. But it will rise 2 percent after inflation, rather than the 8 percent that Mr. Reagan had sought. The difference is $18 billion.

Contrary to the impression that you may have received from the administration, Congress has not succumbed to the temptation to lard the social programs with large increases. The total budget for all nondefense spending in this resolution is a modest $2.3 billion larger than Mr. Reagan's original proposal of more than $700 billion. That's an increase of three-tenths of one percent.

There was some shifting around of priorities -- which was inevitable and, in most instances, highly desirable. Congress rightly refused to inflict the large cuts in student aid that Mr. Reagan had wanted. It increased the money for hospital care for the elderly and the poor more than he wished. Generally it compensated by making its own cuts elsewhere.

One of them, unfortunately, is the deep cut in foreign aid. That is going to make serious trouble, hampering the country's ability to support democracies and respond to emergencies abroad. This defect is a conspicuous lapse in a generally splendid job of budget-writing.

Under this budget resolution, Congress appears to have the 1987 deficit low enough to avoid the drastic cuts across the board that the Gramm-Rudman-Hollings law was to have imposed. If that is the case, it won't make any difference, at least for the coming year, whether the Supreme Court voids the automatic trigger in G-R-H. The president's February budget would certainly have triggered the G-R-H cuts. Among its other services this skillful congressional revision promises to avoid all the inequities and the wild confusion that those cuts would threaten.