Cutting this year's defense budget now that it has been approved by Congress is a bad idea, two senior members of the House Armed Services Committee warned yesterday.

Rep. William L. Dickinson of Alabama, ranking Republican on the committee, and Rep. Charles E. Bennett (D-Fla.), chairman of its seapower subcommittee, said reductions in the defense budget would start an unraveling process that would undercut President Reagan's effort to rearm America.

Dickinson stressed that rather than slash this year's defense budget, as the administration now seems disposed to do to hold down the deficit, any cuts should "be prospective in nature." Bennett said the problem is not too much money for defense but too little revenue because of the hastily passed tax reduction bill.

"It would be tremendously unwise" to start hacking away at this year's $222 billion defense budget, Dickinson said. Besides beginning an unraveling process, it would send the wrong signal to allies being urged to increase their defense spending, he said.

When asked if he would refuse to vote for fiscal 1982 defense cuts if Reagan advocated them, Dickinson hedged, saying he would have to study their size and impact before making a judgment. He is scheduled to meet with Reagan Tuesday, presumably to hear how much the president wants to cut defense spending below previous projections in fiscal 1982, 1983 and 1984 to help balance the federal budget by 1984.

"If push comes to shove," Dickinson said, "I'd rather see the target date for balancing the budget delayed for one year" rather than cut this year's defense budget.

Bennett said, "The problem is not that defense spending is too high, but that the tax cut bill recently rammed through the Congress has placed us in a posture where our revenues are going to be reduced to the point where we can't afford what we need to defend our country.

"This Congress and this president," Bennett continued, "were mandated by the American people 10 months ago to restore this country's military might. We have no right to shrink from that responsibility."