THE BUDGET that just shot through Congress is a fake. Its too much to expect complete arithmetical rectitude in an embittered and exhausted lame duck session. But the second budget resolution goes well beyond the usual bounds of rubbery estimating. The revenue figure is conveniently too high, and the spending limit is unrealistically low. This budget claims that, by the end of the fuscal year, next September, the deficit will be $27.4 billion. On the contrary, it's likely to be nearly twice as high. If you count the off-budget spending -- and you ought to count it, since the difference between off-budget and on-budget spending is a mere legal fiction -- the true deficit will be almost three times the resolution's cheery promise. A better estimate would be slightly over $70 billion, just like last year's deficit.

The Democrats have succumbed to the normal and natural inclination to embarrass the incoming Reagan administration. Mr. Reagan said that he could cut the budget where all others have failed, the argument goes -- so let him cut it. Budget philosophy has always been a prolific source of fraudulent accountancy. Mr. Reagan and his advisers would be wise to shift their attention from the current year's deficit to longer strategies for the control of spending. The federal budget has developed immense internal momentum, and changing its direction is not going to be done quickly, regardless of presidential intentions.

By Inauguration Day nearly four months of the fiscal year will have passed. To meet the congressional spending limit would require reductions from present spending levels on the order of $15 billion by September. But since those reductions would have to be stuffed into the second half of the year, the administration would have to cut at a rate approaching $30 billion a year -- and more, if there are to be increases for defense.

The federal budget constitutes one-fifth of the national economy, and any change in the economy is inevitably going to affect the budget. President Carter learned to his grief that those impacts can be severe. If inflation or interest raters should be higher than the new administration expects, or if there should be continued recession next year, deficits will automatically widen. If there should be another sharp rise in oil prices, or another bad grain harvest abroad next year, that will widen the deficit. Where the new administration makes mistakes or flinches from umpleasant choices, that too will show up in the deficit. But in fairness to the members of the Reagan administration, you have to note that the point from which they begin is not the $27.4 billion deficit in the resolution that Congress has now sanctimoniously enacted. It's that ominously larger figure, which you will find nowhere in the resolution, of $70-plus billion.