Beefing up the Saudi-Arabian military by making its F15 fighters more lethal is in Israel's long-term interest, Chairman John G. Tower (D-Tex.) of the Senate Armed Senate Committee said yesterday.

In the wake of the collapse of the Shah's government in Iran and the Soviet invasion of Afghanistan, Tower said, Saudi Arabia and Perisan Gulf states are determined to strengthen their defenses. If the United States does not sell arms to them, he said, they will turn to other countries.

Israel would be better off, the chairman contennded, if the United States went ahead and carefully upgraded the Saudis' F15s than if Saudi Arabia bought fully armed French Mirage 4000 warplanes.

The Reagan administration is seeking congressional approval of a plan to add advanced air-to-air misiles and fuel tanks to the Saudi F15s, but not the bomb racks the Saudis also requested. Israel's fears about the increased capabilities of the Saudi F15s have stirred up considerable congressional opposition to the plan.

"I don't think it will be easy to defeat a resolution of disapproval," Tower conceded in discussing the F15 issue. But he said opponents should recognize that "the nature of the threat has changed radically over the last three years."

Before the fall of the shah and Soviet invasion of Afghanistan, the Carter administration promised Congress that the Saudi F15s would have limited air-defense capabilities.

The Saudis have no intention of going to war against Israel, Tower said, "They know if they launch an attack against Israel, they'll get clobbered."

Tower, at a breakfast session with reporters, discussed these topics:

Secretary of State Alexander M. Haig Jr. -- He "did precisely what he was supposed to do" in declaring that he was in charge shortly after President Reagan was shot on Monday. Tower said that Haig "is highly regarded on our side of the aisle."

Defense budget -- The Regan administration rewrite of President Carter's fiscal 1982 defense budget "is just about right." Tower predicted that the committee will approve a total just slightly less than the recommended one of $222 billion.

Navy commuications systems -- "We still have pending a little chat" on why the Navy reversed itself last week by stating that it could do without the extremely long frequency (ELF) grid for sending messages to submarines.

The communications net started out as Project Sanguine. It was to be built near Clam Lake, Wis., but environmentalists kicked up a storm of protests.

In the last few years, the Navy has concentrated on selling the administration and Congress on the idea of linking two experimental sites, one in Michigan and the other the original Wisconsin facility, into a single working system. But last week the Navy called off that campaign in a switch that Tower said caught him by surprise.