After an emotional 2 1/2-hour debate last night, the House voted by an overwhelming 303 to 95 to deny federal student loans to youths who don't register for the draft.

Nearly 700,000 nationwide are believed to have avoided registration since the draft was reinstated for 18-year-old males in 1980. It is not known how many might otherwise be eligible for student loans.

The amendment to the defense authorization bill was sponsored by Rep. Gerald B. H. Solomon (R-N.Y.). The Senate approved similar language in May, so the provision is virtually certain to become law.

Said Rep. Thomas F. Hartnett (R-S.C.), "People who want to receive financial assistance from this government should abide by the laws of the land."

Opponents of the amendment, led by Rep. Paul Simon (D-Ill.), argued that students should be allowed exemptions from the ban on loans if they have religious or moral objections to military service. Simon said the amendment would "cause chaos on the campuses" and violate the tradition of conscientious objectors, upheld through World Wars I and II.

Rep. Barry Goldwater (R-Calif.), a long-time opponent of the draft, said the Solomon amendment discriminates against the poor because "if you are wealthy enough not to need student loans, you don't have to register and you can still go to school."

Among Washington area members, Maryland Democrats Michael Barnes and Steny Hoyer voted against the Solomon amendment, while Republican Marjorie Holt voted for it. The entire Virginia delegation voted for it.

In another amendment to the $177 billion defense authorization bill, the House voted to prohibit spending on weapons that would violate the unratified SALT II treaty unless President Reagan invokes "the supreme national interest."

Reagan has said he would not undercut the strategic arms limitation treaty "so long as the Soviet Union shows equal restraint." However, the administration has preferred not to be bound by any legal obligation. Reagan opposed SALT II during his election campaign and has begun his own Soviet arms reduction talks.

Rep. Les Aspin (D-Wis.), author of the SALT amendment which was accepted by the Armed Services Committee and approved yesterday by voice vote, said it might affect the so-called "Dense Pack" basing of the MX missile. Some arms control experts contend that Dense Pack, a recent proposal for tightly grouping the new missiles to protect them from Soviet attack, would violate SALT II.

Another amendment approved overwhelmingly would allow divorce courts to award a divorced spouse a portion of a serviceman's retirement pension. Sponsors Patricia Schroeder (D-Colo.), William Whitehurst (R-Va.) and Kent Hance (D-Tex.) argued that military men have been divorcing their wives after many years, leaving them with no financial benefits after they had followed their husbands from post to post, often sacrificing their own careers.

A measure backed by the Northeast-Midwest Coalition to channel up to $5 billion a year in non-strategic purchases to high unemployment areas passed 237 to 170.

Other amendments would: force the Defense Department to send Congress reports on major weapons systems whose costs increased by 15 percent; prohibit purchase of essential equipment from foreign manufacturers if it makes the United States dependent, unless the defense secretary notifies Congress in advance; and require Defense Department maps to show Estonia, Latvia and Lithuania as separate countries from the Soviet Union.