President Reagan last night reaffirmed his determination not to let terrorist attacks drive U.S. Marines out of Lebanon, but he gave no indication that the administration has any new plans for giving the Marines greater protection or for bringing peace to Lebanon.

Reagan devoted the first part of his nationally televised speech to Lebanon, speaking in the aftermath both of last Sunday's devastating bomb attack against the Marines and of a major review of Mideast policy that he conducted last week with his top advisers.

After the policy review, there had been hints that the administration was preparing new initiatives to break the stalemate in Lebanon and get the president's stalled proposal for resolving the Arab-Israeli conflict back on track. The heavy death toll from the bombing seemed to underscore the need for fresh approaches that would ward off a new national debate about the wisdom of keeping the Marines in Lebanon.

But if the administration has any new ideas about how to resolve the Lebanon crisis, Reagan gave no sign of it last night. Instead, he reiterated in general terms the administration's intent to keep supporting Lebanese President Amin Gemayel's efforts to reach a negotiated settlement of the civil war there.

He also said the United States "will work even more closely" with its allies in the multinational force--France, Britain and Italy--"for the rebuilding of a national consensus" within Lebanon. And he vowed to give the Marines "the greatest possible protection."

None of his remarks went beyond a restatement of existing U.S. policy, and his lengthy recital of the reasons the United States must stick to its present course in Lebanon was only an elaboration of the remarks he made Monday, the day after the bomb attack.

As he did then, the president insisted that the Mideast, with its energy resources and strategic location, "are vital to us and to world peace." He insisted that Lebanon is the key to stability in the region and said: "If that key should fall into the hands of a power or powers hostile to the free world, there would be a direct threat to the United States and to our allies."

Reagan also contended that the Marine presence is accomplishing its purpose of strengthening Gemayel and forcing the Moslem and Druze factions fighting his Christian-dominated government to seek a negotiated solution.

"The multinational force was attacked precisely because it is doing the job it was sent to do in Beirut--it is accomplishing its mission," Reagan asserted.

The president said he soon will announce a new special Mideast envoy to replace Robert C. McFarlane as his personal mediator in the Lebanese conflict. But he gave no indication of what steps are being contemplated to induce Syria to agree to withdraw its forces from Lebanon.

Instead, he omitted the conciliatory rhetoric that U.S. officials had addressed to Syria until recently and spoke about President Hafez Assad's regime in tones that seemed to reveal increasing U.S. frustration over Syrian intransigence. He noted that Syria "makes no secret of its claim that Lebanon should be a part of a greater Syria," and he added:

"Today, Syria has become a home for 7,000 Soviet advisers and technicians who man a massive amount of Soviet weaponry, including SS21 ground-to-ground missiles capable of reaching vital areas of Israel."

He also charged that "Syria reneged on its promise" to cooperate with plans for a withdrawal of all foreign forces after Israel and Lebanon reached agreement last May for the pullout of Israeli troops.

" 'If our Marines must be there,' I am asked, 'why can't we make them safer?' " the president said. However, despite expectations that he might announce specific steps, such as the broadening of the Marines' sector in the Beirut airport area to include more defensible terrain, Reagan said only that the Marine Corps commandant, Gen. Paul X. Kelley, who returned from Beirut yesterday, "will be advising us on steps we can take to improve security."

"Brave young men have been taken from us," Reagan said. "Many others have been grievously wounded. Are we to tell them their sacrifice was wasted? They gave their lives in defense of our national security every bit as much as any man who ever died fighting a war. We must not strip every ounce of meaning and purpose from their courageous sacrifice."

Reagan said that Dr. Kenneth Morrison, father of a Marine in Lebanon, had sent him a message saying, 'My son has chosen the acceptance of responsibility for the privilege of living in this country.' " The president added, "I was thrilled for him to learn today that his son, Ross, is alive and well and carrying out his duties in Lebanon."

He said there is "strong circumstantial evidence" that the Marines were attacked by the same terrorist group that bombed the U.S. Embassy in Beirut last April.

"Those who directed this atrocity must be dealt justice," he asserted. "And they will be."

Again, however, he gave no specifics, despite reports from Paris earlier yesterday that Secretary of State George P. Shultz had discussed at a meeting with the French, Italian and British foreign ministers possible reprisals against the suspected terrorists.

A senior administration official who briefed reporters before Reagan's speech refused to elaborate on what had been discussed at the Paris meeting. The official, who declined to be identified, also would not discuss reports that the United States has evidence linking the bombings to a pro-Iranian Shiite Moslem group known both as Islamic Amal and as the Party of God, with headquarters in a part of Lebanon under Syrian control.