President Reagan, in a major foreign policy address Wednesday, will charge that the Soviet Union is building a new mobile nuclear missile "clearly designed to strike first" against the United States, National Security Adviser Robert C. McFarlane said today.

In an address by Reagan to the European Parliament that McFarlane, in an unusual move, made public in substantial detail today, the president also will disavow any intention to seek military superiority over the Soviets -- a goal he pledged in his 1980 campaign and reiterated in the early years of his presidency.

McFarlane said "the whole tone of the speech is toward accommodation and solving problems" with the Soviets, coupled with a warning in a major European forum about the new Soviet SSX24 intercontinental ballistic missile.

Although U.S. defense officials have known about and publicly discussed the SSX24 for about two years since flight testing began, McFarlane said Reagan was raising questions about the missile now because it has not been deployed and thus is "still susceptible to influence in arms control talks."

The extensive disclosure of Reagan's text a day in advance for the second day in a row appeared to reflect a White House campaign to capture the headlines in Europe with Reagan's message even before the speech is delivered in Strasbourg, France.

It is highly unusual for White House officials to release major portions of a presidential address for publication the day before the speech is made. The tactic appeared certain to focus widespread attention in Europe on Reagan's speech as he nears the end of his 10-day trip.

Reagan will charge that the Soviet Union has been building weapons "clearly designed to strike first and thus to disarm their adversary," McFarlane said.

While Reagan previously has not charged this directly, members of his administration have voiced concern about the new Soviet missile.

Specifically, he added, Reagan refers to Soviet testing of the mobile, 10-warhead SSX24 missile which would have a first-strike capability and also "the potential to avoid detection, monitoring or arms control verification."

The new missile "is a problem, a new dimension of the strategic equation, which is particularly difficult to deal with," McFarlane said.

If the missile is deployed, he said, "one can imagine the time in which the president would be faced during a crisis with not knowing how many or the location of Soviet strategic power.

"This is an intolerable condition," he added, giving the Soviets "an apparent first-strike capability" that could leave the United States "vulnerable to coercion and nuclear blackmail."

According to the 1985 edition of "Soviet Military Power," published by the Defense Department, the SSX24 is one of two new solid-propellant intercontinental ballistic missiles being developed by the Soviets. The other is the single-warhead SSX25.

Both are said to be "well along in their flight test programs," and a mobile version of each "will be deployed" eventually, the report said.

Under the 1979 SALT II agreement, each side is allowed to test and deploy one new type of "light" land-based missile. But the Reagan administration has charged that a second missile violates that accord and also has raised questions about how "light" in size and weight the new Soviet missiles are. The U.S. Senate never ratified SALT II but both sides have said they would abide by it.

McFarlane said today that Reagan's concern was not focused on the SALT II treaty, but directed at the capabilities of the new missile, which is expected to be shuttled around the Soviet Union on rail cars and hidden in mountain tunnels.

The administration has estimated that the SSX24 would be deployed in silos next year and on trains within one or two years.

The United States is also building a 10-warhead ICBM, the MX, but a plan for mobile deployment was scrapped by the Reagan administration. Reagan's current plans call for deployment of 100 MXs in existing Minuteman missile silos, but Congress has approved production money for less than half this total. Reagan's existing plan also looks forward to eventual deployment of small, mobile single-warhead Midgetman missiles.

Although Reagan will say U.S. stratetic superiority "eroded" in the 1970s, he will add that it is "not my view" that the United States should seek to restore nuclear superiority over Moscow, McFarlane said.

"The United States does not seek to undermine or change the Soviet system, nor to impinge upon the security of the Soviet Union. At the same time, it will resist attempts by the Soviet Union to use or threaten force against others or to impose its system on others by force," Reagan will say.

Reagan had called for U.S. military superiority over the Soviets in his 1980 campaign, saying the United States must be "second to none." He held this view in the early years of his first term but later muted it. An effort was made by conservatives to put a call for U.S. superiority in the 1984 Republican Party platform, but the White House successfully pressured the GOP platform committee not to include it. A compromise called instead for maintaining "technological superiority" over the Soviets in most military matters and "qualitative superiority" in maritime strength.

After reiterating the importance of U.S., British and French nuclear modernization, Reagan will say in his speech that the Soviets do not "share our view of what constitutes a stable nuclear balance."