By his choice of George P. Shultz as secretary of state, President Reagan has indicated that he will follow a policy of continuing toughness but probably less verbal bristling in U.S.-Soviet relations.

In what both superpowers agree is a decisive stage in the Reagan administration's efforts to restrain Soviet military power, the president has, in effect, exchanged one moderate conservative for another, although the circumstances of outgoing Secretary Alexander M. Haig Jr.'s departure strongly suggest that the White House itself is determined to control the style and the substance of U.S. foreign policy.

The nomination of Shultz gives no comfort to the stridently anticommunist right wing of Reagan's supporters, which has been registering dismay over "the capture" of Reaganism by "State Department bureaucrats."

It saw Haig as "a sheep in wolf's clothing," who operated as a pragmatic moderate beneath a screen of challengingly anti-Soviet rhetoric.

By that measure, Shultz is worse news to the extreme right. His style is distinctly nonbelligerent. Shultz's views on many specific issues in foreign affairs -- with the notable exceptions of trade and Middle East policy and trade -- are generaly little known.

In his extensive experience in international economic crises, Shultz has been a reconciler, not a confrontationalist, operating largely behind the scenes with firmness and adroitness.

Entering the administration at this stage, Shultz presumably shares the president's basic contention, which is that there is a need to augment American military power to bargain more forcefully with the Soviet Union.

What he does not possess, however, is the knowledge of military affairs, strategy and arms control that Haig acquired as a commander of the North Atlantic Treaty Organization and earlier as deputy national security adviser in the White House.

This troubles many specialists here and abroad, especially at this point in precarious U.S.-Soviet relations, when a maze of interrelated arms control and East-West trade issues requires day-to-day decisions.

In Geneva Tuesday, the United States and the Soviet Union will begin the most significant arms control negotiations in this administration, on intercontinental nuclear weapons: the strategic arms reduction talks or START.

These negotiations replace the strategic arms limitation talks of the Nixon-Ford-Carter years that produced the unratified SALT II arms control accord that this administration has assailed as "fatally flawed," even though it continues to be honored.

At the same time, parallel U.S.-Soviet negotiations have been under way in Geneva since Nov. 30, to limit European-based intermediate-range nuclear missiles. The overlapping negotiations, bound to confuse nonspecialists, may ultimately merge.

Peter Osnos of the Washington Post Foreign Service reports from London a British concern that until Shultz learns the details, the dominant official in the Reagan administration on arms control will be Defense Secretary Caspar W. Weinberger, who is considered "to be more hawkish than Haig."

Haig and Weinberger were on opposite sides of an intense administration debate in preparing U.S. strategy for START.President Reagan decided to put in the "first phase" for a settlement the comparatively less demanding terms advocated by the State Department, and the Weinberger-supported demands in the "second phase." Even so, the first-phase demands would require major restructuring of Soviet nuclear forces and have been assailed by Moscow as grossly lopsided.

Shultz's special advantages to the Reagan administration right now include the experience he can bring to bear on the second half of a two-track strategy: to put economic pressure on the Soviet Union to reinforce American demands, both in the arms control negotiations in Geneva and on such other fronts as Poland.

Many Western Europeans indignantly believe that the Reagan administration double-crossed its allies in its desire to put economic pressure on the Kremlin. This turns on Reagan's decision June 18 to ban the sale of American technology, or equipment produced abroad under license from U.S. firms, for use in the planned 3,700-mile Soviet natural gas pipeline from Siberia to Western Europe.

The Europeans were led to believe that the Reagan administration had given up trying to block the pipeline and to persuade its allies to join an "economic war" against the Soviet Union. The widespread understanding in Europe was that the United States, instead, had agreed to hard-bargained but imprecise language at the recent Versailles economic summit conference for curbs on Western trade credits for the Soviet Union as a tradeoff for acqueiscing to the pipeline.

Haig strongly argued the European case in the White House -- and lost. It was not that Haig was any less in favor of putting economic pressure on the Soviet Union. But he carried the operational responsibility of trying to sustain both the negotiating and the pressure tracks of administration policy, and an explosive backfire in Europe over the Reagan pipeline decision was inevitable.

The pipeline dispute was one of the issues behind Haig's charge, in his letter of resignation, that the administration had swerved from the course of "consistency, clarity and steadiness of purpose."

Failure by the administation to calm the uproar over the pipeline could jeopardize its entire Soviet strategy and undermine allied plans for deploying in late 1983 Pershing II missiles and cruise missiles targeted on the Soviet Union.

This planned deployment in Western Europe is the allied bargaining card in the Geneva negotiations on intermediate-range nuclear weapons and therefore is also important U.S. leverage in the closely related START.

One of Shultz's initial tasks presumably will be to resolve this dangerous split. Like Haig, Shultz has high prestige among Western European leaders; while he lacks Haig's strategic experience or ties, Shultz had direct involvement in events leading up to the pipeline decisions. He did advance work for Reagan for the Versailles conference.

Shultz is on record, however, as ridiculing at least some concepts of trade as a political weapon. In an article in Business Week in May, 1979, Shultz scorned the idea that trade "can be turned on and off like a light switch" to compel foreign governments to change policy.

He singled out shifting decisions by the Carter administration, including a sequence over whether to permit an American company, Dresser Industries, to sell a $150 million drill-bit plant to the Soviet Union. The deal was finally permitted.

Shultz stressed the need for a "predictable set of rules" to avoid domestic and foreign confusion -- a stand very much like Haig's. Administration sources said they assumed, however, that Shultz accepted Reagan's pipeline decision.

What remains to be seen is whether reconciler Shultz can harmonize his views with those that Treasury Secretary Donald T. Regan expressed early this month on linking Soviet trade and arms control: "As we begin the START talks they [the Soviet Union and its allies] will see a tradeoff proposal. Make some type of accommodation with us in the field of strategic arms, or else watch your own weak economies weaken further."