The nation's largest veterans' organization, the American Legion, today voted to support the strategic arms limitation treaty, provided certain amendments and increased military spending are forthcoming.

In a carefully worded statement of conditional support, the Legion urged neither rejection nor approval of the pending arms treaty by the Senate. Rather, the organization sought to give its leaders room to negotiate with Senate committees while delaying a final position on the treaty.

"We're not out to kill the treaty," said James B. Hubbard, deputy director of the Legion's headquarters staff on national security and foreign policy, "This gives us enough flexibility to deal with the political realities in Washington, D.C."

Hubbard would not say what combination of treaty changes and increased military spending might win ultimate Legion support.

Compared with the organization's 1977 flat opposition to the Panama Canal treaties, today's vote represents something of a gain for the Carter administration. But the significance of today's action is unknown.

The Legion, for example, eventually saw the Panama Canal treaties ratified. Hubbard declined to say whether that experience contributed to Legion flexibility on SALT II.

But the administration had courted the Legion, which has more than 2.5 million members. With leaders of other groups, its commanders were invited to White House and Pentagon briefings on the intricacies of the second arms limitation treaty the United States has negotiated with the Soviet Union.

Today's stance was adopted unanimously on a voice vote by an unknown number of delegates at a sparsely attended session during the Legion's 61st annual national convention here. Many of the legionnaires in the Sam Houston Coliseum seemed more preoccupied with the election of national officers than with nuclear policitics.

The Legion's SALT II position was, in the words of one leader, not unlike that of former secretary of state Henry A. Kissinger, who has also given SALT qualified support.

The formal resolution enunciates support for "any SALT agreement which is genuinely equitable, which actually halts the nuclear arms race, reduces tension and guarantees full compliance by both sides."

At the same time, it oppose any treaty that leaves the United States inferior in the quantity or quality of nuclear weaponry.

Administration officials, including Vice President Mondale, in a string of public appearances this summer, have argued that the proposed SALT agreement meets such requirements, and to the benefit of the United States.

Hubbard, however, interpreted for puzzled reporters the resolution he helped draft as one that reserves, for the Legion the right to make a final judgment for itself after lobbying for amendments to the SALT treaty.

The Legion today saw room for these improvements in the treaty.

The explicit incorporation into the treaty of U.S. - Soviet agreements on production of the Soviet Backfire bomber.

The inclusion of other agreements and understandings in documents.

Insistence by the Senate on the right to review any executive agreements extending certain portions of the treaty past 1981.

In a series of "whereases," the Legion also expressed concern over recent Soviet behavior, the decline of the United States as a world power and the size of the Soviet heavy missile launcher fleet.

"We'll support it if they build up the military," said Joseph H. Ellinwood, chairman of the Legion committee on SALT II, said of the treaty.

"But we want definite guarantees, positive action, not lip service."

Both Ellinwood and Hubbard said that without an increased Pentagon budget and some changes in the treaty they would oppose final approval. But for now, and when the Legion testifies on the treaty next month, they will offer conditional support.

"We want to be in an influential position," Hubbard said. "It's flexible. But if that treaty is not amended, I will not recommend that my organization support it. We want to see what the Senate Foreign Relations Committee does (with modifications)."