Former defense secretary Harold Brown called yesterday for cutting military spending by $50 billion in fiscal 1985 and 1986 as part of an effort "to save us from an economic crisis and retain sufficient military capability."

Brown, who as secretary in the Carter administration had pushed for defense increases but smaller ones than the 7 to 8 percent a year President Reagan is recommending, also said taxes have to be raised and entitlement programs cut to reduce projected $200 billion deficits, which he said should be slashed by more than half.

Focusing on the Pentagon budget, Brown told a group of reporters that "you have to make reductions now" in Reagan's requests or it will be too late to affect spending levels in fiscal 1985 and 1986. He recommended a $25 billion reduction in expenditures for each of those two years.

Asked what weapons he would cancel, Brown listed the $30 billion B1 bomber, $6.8 billion the Navy is requesting for two nuclear-powered aircraft carriers, and the Marines' $10.6 billion AV8B jump jet.

Reagan's five-year rearmament program -- calling for $1.6 trillion to be authorized and $1.5 trillion actually spent in fiscal 1983 through 1987 -- will run into real trouble next year, Brown predicted.

"I think there will be a very substantial crunch next year as people realize that fiscal 1985 and 1986 budgets are likely to show over $200 billion deficits, even if there is more of a recovery than I think is likely to happen."

To signal the financial community that the federal government will not be borrowing heavily in fiscal 1985 and 1986, thus driving up interest rates, Brown said the deficit has to be cut "by over $100 billion, say $120 billion."

In a broad-ranging discussion, Brown made these other points:

* Reagan's recommended Dense Pack deployment, bunching MX intercontinental missiles close together outside Warren Air Force Base in Wyoming, is badly flawed. About 1,000 more holes ought to be dug for the 100 missiles, both to reduce vulnerability and to keep the deployment plan within the terms of existing arms control treaties.

* Our submarines are relatively safe today, but by the 1990s the Soviets might turn to the tactic of exploding a series of one-megaton bombs in ocean areas where sensors indicated American subs were hiding. Thus the United States must continue to have land-based missiles like the MX as well as submarines and bombers.

* The current Marine Corps presence in Lebanon may evolve into a longer stay than is advisable at this point in Middle East peace negotiations. American troops are not well suited to the "constabulary" role that has been thrust upon them in Beirut.