WHILE THE budget resolution is something of a triump for President Reagan, it is also a warning of the extradordinary difficulties ahead. Cutting the budget as everyone expected. To keep within Mr. Reagan's target figures for next year, Congress has had to pledge itself to severe cutbacks even in areas, such as nutrition and job training, that are hardly frivolous or marginal public responsibilities. Yet even the budget resolution, as it emerged from conference on a wave of votes supporting the president, is -- using the conferees' figures -- only 2 or 3 percent below present spending. If it were recomputed with less tendentious figures, it would show the same spending level as this year or perhaps a little higher.

By its decision to use the stretched, bent and slightly fake figures in this budget, Congress is steering itself toward serious trouble next fall. The temptation was, to be sure, extreme. A substantial majority in both houses, including some of the most adament fiscal conservatives, felt it necessary to slip back a little of the money here and there that the administration had purposed to cut. But those large majorities also wanted to stay within the totals that the president had set and, in the case of the deficit, to go him one better.

For those who succumbed to the overwhelming desire to put a little of that education aid back into the Reagan budget without increasing the deficit, there was at least a temporary solution. Why not raise the revenue estimate by a few billion dollars? Those things are never entirely precise anyway. To get some of the food money back in, why not merely assume that interest rates are going to fall sharply next year? If spending is still several billion dollars too high, the next step is clearly to throw out the Strategic Petroleum Reserve and hope that it can eventually be financed by some route that won't show up in the budget. The budget resolution, as it now stands, sets a spending total of $695 billion. Next year's deficit is not the stated $38 billion, but $50-plus million -- better than this year's $60-plus billion, but only slightly better.

This resolution is, of course, only the first of two. This one sets limits for the congressional committees in the months ahead, but the second resolution, next September, sets legally binding limits on spending and the deficit. At that point, the practice of systematic understatement becomes much more difficult to maintain. Then Congress will feel another kind of temptation -- to throw up its hands, declare that the performance of the economy has been unexpectedly poor, and announce that the budget is out of control. That kind of theater is not helpful to public confidence in the budget process.

Speaking of confidence, the amount of arithmetical fudging in this budget resolution does not generate much confidence that Congress is prepared to continue the process of cutting these programs on a rigid schedule in the years ahead. The Reagan fiscal plan requires further large reductions there to keep step with increases in defense and the drop in the tax rates that the president seeks. But the message from the Capitol is that, while spending cuts are possible, they are getting progressively harder.