President Reagan yesterday won an important battle but not the war over the MX missile as the House Appropriations Committee defeated, by a 26-to-26 tie vote, an amendment to prevent production of the land-based intercontinental nuclear weapon.

"We're not jumping up and down and claiming the victory because we know it's going to be a tough battle on the floor," Reagan said in Sao Paulo, Brazil, on his tour of Latin America.

"The floor vote will be just as tough as this one. We're not out of the woods yet," said Rep. Jack Edwards (R-Ala.), Reagan's field general in the House for the MX, which has become a major test of congressional support for the president's defense buildup.

"We'll do well on the floor," vowed Rep. Joseph P. Addabbo (D-N.Y), who narrowly lost his attempt yesterday to delete $988 million for production of the MX from the $231.6 billion defense appropriations bill later approved by the House committee.

It appeared for a moment that Addabbo had won the dramatic vote in the overcrowded Capitol hearing room when Rep. Bill Alexander (D-Ark.) passed during the call of the roll. But Alexander than cast his vote with the president and against Addabbo to create the tie that defeated the amendment.

Ten Democrats joined 16 Republicans to swing the vote for Reagan, while Addabbo won over 20 other Democrats and five Republicans. Among the wavering Republicans whom Reagan had called to lobby this week was Rep. Virginia Smith of Nebraska, who also ended up voting the way he wanted.

Edwards said the plan is to bring up the defense money bill on the House floor on Tuesday, send it to the Senate by Thursday and hold a House-Senate conference to resolve differences over it the following week to avoid leaving the Pentagon with less money through a continuing resolution.

The MX will be contested both on the House floor and in the Senate, where Sen. Ernest F. Hollings (D-S.C.) claims to have enough votes to derail Reagan's plan to deploy 100 MX missiles, each with 10 nuclear warheads, in a closely spaced "Dense Pack" formation outside Warren Air Force Base near Cheyenne, Wyo.

The Pentagon contends Dense Pack would require attacking Soviet warheads to fly so close together that they would destroy each other when they exploded over the MX field. But Addabbo argued yesterday that "nobody knows what Dense Pack is."

"How could you go back to your districts and say we are going to give a gift of $988 million without us knowing where we're going?" he asked colleagues on the House Appropriations Committee. He said deleting the production money would not necessarily stop the MX from being deployed on schedule in 1986, but would be telling the Pentagon, "When you're ready to tell us how and when the money will be spent, we'll take another look at it."

Edwards replied that "we're already late" in getting the MX built and that sticking to the president's schedule would show "above all else, that we're going to keep the pressure on to keep the Soviets at the bargaining table" in nuclear arms reduction talks in Geneva.

After Addabbo's amendment failed by the narrowest possible margin, the committee approved by voice vote two amendments to forbid the Pentagon from spending money to produce MX or to start on Dense Pack until March 15. Edwards said the Pentagon had approved these amendments in advance.

Asked if this would make it easier for opponents of the MX to mobilize their forces, Edwards said the restrictions would make the program "more credible" by assuring skeptics the administration was not rushing ahead with Dense Pack before evaluating it thoroughly.

Challenges to other major weapons systems in the defense money bill were beaten back comparatively easily in the committee voting yesterday. An Addabbo amendment to delete $3.6 billion for one of the two Nimitz nuclear aircraft carriers in the bill failed by 34 votes to 18. Another Addabbo effort to deny $3.4 billion for the B1 bomber failed 29 to 15 on a show of hands.

Edwards stressed that the $231.6 billion bill approved yesterday is not the work of "spendthrifts," declaring it is $17.9 billion less than Reagan requested originally. He termed this "the largest budget authority cut in 22 years."

Reagan was most worried about losing a crucial battle of the MX fight in the House committee. He spearheaded an intense lobbying effort, calling committee members from the White House last weekend and then from Brazil.