Washington — The Reagan administration is carefully laying the groundwork this week for coming arms control talks with the Soviet Union. Defense Secretary Caspar W. Weinberger - just back from a trip to Europe, where he sought to assure allies of United States resolve in reducing nuclear weapons - is making a pitch to continue this country's military buildup as a means of inducing the Soviet Union to negotiate.
Secretary of State George P. Shultz left last night for meetings in London, Brussels, and West Germany. Along with his new special arms control adviser, Paul Nitze, Mr. Shultz also seeks to ease allied concern for such controversial US programs as the strategic defense (''star wars'') initiative. The administration will insist that space-based missile defenses, now in the research stage, not be thrown out before ''umbrella'' arms control talks on offensive nuclear weapons begin.
Here at home, the administration faces a difficult task in convincing Congress that such potentially powerful bargaining chips as the MX missile and strategic defense should not be killed just as talks with Moscow are about to restart. That fight begins this week with the shaping of the administration's fiscal 1986 military budget.
While there is a new, expectant mood in Washington, senior administration arms control officials and outside experts are also cautioning against expecting too much from the Jan. 7-8 talks scheduled between Mr. Shultz and Soviet Foreign Minister Andrei A. Gromyko. Indeed, it is being said, progress may not necessarily be measured in terms of a signed and formal arms control agreement of the type worked out in the past.
Of the meeting in early January, former White House national-security adviser Brent Scowcroft said, ''I wouldn't expect much of anything of substance to come out of it.
''I would expect a sorting out of what an (arms control) agenda might look like,'' explained Mr. Scowcroft, who chaired the Commission on Strategic Forces named by President Reagan to sort out the complexities of strategic doctrine and weaponry. ''That's more than enough for Shultz and Gromyko to take up.''
Over breakfast Monday, Scowcroft warned reporters ''not to put all your weight into scoring it.'' At the same time, the retired Air Force lieutenant general also strongly praised Mr. Nitze, who has been leapfrogged in authority over other administration officials who often squabbled over negotiating positions.
Scowcroft, who himself had been mentioned as a possible arms control ''czar, '' said, ''Paul Nitze is outstanding. He probably knows more about the details of arms control - where all the bodies are buried, what the positions of the Soviet Union are - than anybody else.''
Articles in the winter issue of Foreign Affairs magazine by Nitze and Kenneth Adelman, head of the US Arms Control and Disarmament Agency, indicate that the United States is not in any hurry to achieve a comprehensive arms control agreement.
The time may have come, Mr. Adelman writes, when ''arms control without agreements'' should be worked for.
''In simple terms, each side would take measures that enhance strategic stability and reduce nuclear weapons in consultation with each other, but not necessarily in a formalized, signed agreement,'' says Adelman. ''Adopting this approach of individual, parallel restraint could help avoid endless problems over what programs to exclude, which to include, and how to verify them.''
In a separate article in the same quarterly journal, Nitze calls for a policy of ''live and let live'' with the Soviet Union.
''Constructive discussion could lead to action on one side, concurrent with action on the other,'' he writes. ''Such an action-reaction process could conceivably provide a more solid foundation for progress in Soviet-American and East-West relations than a negotiated set of principles of conduct.''
It is these kinds of atmospherics and subtleties that Shultz and Nitze will be probing in Geneva next month and presumably in follow-up meetings.
In Washington, meanwhile, the Reagan administration faces great difficulty in keeping its most visible strategic modernization program, the 10-warhead MX intercontinental ballistic missile, alive. The MX, which must pass both House and Senate funding tests this spring, lost several supporters in the Senate during the recent election. And Sen. Barry Goldwater (R) of Arizona, the new chairman of the Senate Armed Services Committee, said in an interview last week that he no longer favors the MX.
In the defense budget being crafted at the White House this week, the administration will again ask for more of the controversial missiles. As they did this year, officials will argue that the MX is necessary to help reduce the wide Soviet advantage in large intercontinental missiles. But, concedes a senior Air Force programs officer, ''We're in big trouble on the MX.''