The $1.15 trillion federal budget submitted this week by President Reagan - his last major act in his final 11 days in office - bears little resemblance to what will probably emerge when Congress gets through with it.

In many ways, the budget is vintage Reagan: heavy on military defense, cuts in social spending, proposed elimination of a variety of programs, optimistic revenue forecasts, and a large deficit - $92.5 billion.It is a budget that President-elect George Bush will have to live with, more or less. Between now and Oct. 1, 1989, when the fiscal 1990 budget takes effect, he will be able to emphasize some things, propose others, and work with Congress toward the finished product. But a budget with Bush's own personal stamp on it will have to wait until next year.

How much attention Congress will pay to Bush in financial matters is not clear. Right now, the language on both sides is conciliatory and cooperative. But unhappily, it is not likely to stay that way very long.

Some Democratic leaders already have voiced objections to the Reagan budget, both in the matter of military spending and social programs.

The budget does have serious flaws. Like most federal budgets, including those fashioned by Congress, it contains a great deal of wishful thinking regarding revenues, plus more funding of some programs, and a significant reduction in the deficit - lowering it from about $140 billion to around $93 billion - all without raising taxes.

This wizardry depends on elimination of some 82 federal programs, including the Amtrack subsidy and urban mass transit subsidies; slashing $35 billion from farm subsidies; slowing the growth in funding for Medicaid and Medicare, and freezing the cost-of-living raises in the federal employee retirement program.

Good arguments can be made for many of those cuts, but realistically, Congress won't accept all of them. Many have been proposed in the past and failed.

In addition, the Reagan budget depends heavily on optimistic forecasts of a dynamic national economy, which theoretically would pump vast amounts of new revenue into federal coffers. The budget projects 3.2 percent growth in the Gross National Product and an inflation rate of only 3.8 percent.

Many economists - including the government's own Federal Reserve - think this is altogether too rosy a view and that growth will only be around 2.5 percent for the year.

If the budget makers turn out to have been too optimistic - and forecasts on the debt have consistently been too low in recent years - the 1990 deficit may not fall below the $100 billion mark.

That is the magic number. According to the Gramm-Rudman law, if the deficit is above $100 billion in 1990, there will have to be across-the-board cuts to bring down the figure. That may be politically safer, but it is not the best way to make budget cuts.

By asking for $315 billion in defense spending - that's a 2 percent increase on top of increases for inflation - Reagan may have misread the public pulse as well as Congress.

In fighting federal deficits, the Pentagon no longer can consider itself exempt. Expensive new programs and weapons systems may be particularly vulnerable to congressional budget cutters.

Reagan also proposes raises in spending to fight drug abuse, for prison construction, for big science projects like a space station and the supercollider, to upgrade the air traffic control system, to fight AIDS, and to clean up hazardous and toxic wastes.

A good case can be made for every one of those programs, but where is the money coming from? The time is long past when the nation can indulge in every good cause without first identifying how they are to be financed. In the past, those ideas have been only loosely linked, if they have been linked at all.

The challenge for Congress as it tackles the federal budget is not only to pay for programs, but to eliminate the deficit at the same time. That cannot be done painlessly or with creative budget-making based on wishes.