Mikhail Gorbachev's speech next month marking the 70th anniversary of the Bolshevik Revolution is partly intended to encourage Ronald Reagan's risky de'tente and nuclear disarmament policies by rehabilitating Bolsheviks murdered by Josef Stalin and charging that the Stalin-Hitler pact brought on World War II.

Gorbachev's total break with both Stalin and Leonid Brezhnev that informed officials say he will announce explains the sudden economic reforms in Bulgaria and Poland over the past few days. The Soviet leader is demanding quick change in his Soviet satellite states to cushion the impact of the most sensational break with the past any Soviet leader has ever dared.

If Gorbachev goes ahead with his current plans, his next public move will come at his summit with President Reagan. There he will sign the INF pact, removing intermediate-range nuclear missiles from Europe, and demand speedy strategic disarmament with this real but shrouded goal: splitting Europe and the United States. There is no sign that Reagan is prepared to cope with this political coup.

Sources inside the Soviet Union and beyond its borders foresee a West both stunned and made vulnerable by the radical sweep of what Gorbachev will say on the eve of the Nov. 6 anniversary. To help control domestic reaction, Gorbachev is expected to brief East European and other Communist leaders on Nov. 5, including Nicaragua's Daniel Ortega. He wants their anniversary speeches to duplicate and anticipate his own.

Rehabilitation of Leon Trotsky, Lev Kamenev, Grigory Zinoviev and scores of lesser Bolshevik leaders murdered or sentenced to death under Stalin is bound to have a sensational impact on the West. Gorbachev may go so far as to say that Lenin's revolution was only the first and that now a second Marxist revolution is necessary to correct post-Lenin mistakes. He will castigate Stalin for dealing with the Nazis.

Worried Western politicians privately predict convulsive pro-Gorbachev reactions in parliamentary democracies after Gorbachev makes his big push for a cleansed Marxist Russia to lead the world into ''peace,'' following two years of his domestic glasnost. But few of those Western politicians are found in the Reagan White House or State Department.

Secretary of State George Shultz, with total control of U.S. policy, has planned no new strategies to avoid being carried away by Gorbachev. Instead, he is described by critics as feeding the term-end hunger for de'tente that has seized Reagan.

A foretaste came last Friday following a full hour that West German Foreign Minister Hans-Dietrich Genscher spent with Shultz and National Security Adviser Frank Carlucci. Shultz arranged for an unannounced Oval Office chat for Genscher with Reagan.

Genscher is the leading de'tentist in West Germany's coalition government and the only German leader Shultz looks to for advice on navigating in the turbulence caused by Gorbachev. Shultz does not consult conservative Franz Josef Strauss, leader of the biggest coalition partner in Chancellor Helmut Kohl's Christian Democratic government. That freezes out the German right and gives Genscher's tiny Free Democratic Party extraordinary influence in Reagan's White House.

In their 15 minutes together, Genscher emphasized to Reagan all the positive aspects of what Gorbachev calls his ''revolution,'' giving the negatives a light brush. That shows how the dangers to the Western alliance foreseen by other European leaders seldom penetrate the Oval Office, and never as easily as Genscher's strong de'tentist views.

These dangers were quietly spelled out in a recent speech in New York by French Foreign Minister Jean-Bernard Raimond. He said that the INF treaty, far from ''closing a chapter in East-West relations,'' will be a new base for the Soviets to ''launch a diplomatic offensive to bring about the progressive denuclearization of the American presence in Europe'' and push for an end of all nuclear weapons.

The danger of this, Raimond said, is that ''the abolition of all nuclear weapons is apparently now {also} the objective of the United States.'' This referred to Reagan's pledge, repeated in his CNN interview Saturday, to achieve the ''total abolition'' of all nuclear weapons.

Shultz has apparently failed to warn Reagan that promising an end to all nuclear weapons is interpreted in Europe as promising a return to conventional weapons, where the Soviets are prohibitively superior, as the deterrent to Soviet aggression.

That demoralizes realists among America's European allies. They see the United States turning its back on almost 40 years of successful nuclear deterrence. It plays into the hands of anti-nuclear peace blocs who are enraptured by the Gorbachev ''revolution'' and will probably be ready to carry pro-Gorbachev politics into the streets after his dramatic pronouncements next month.

Celebration of the Bolshevik Revolution three weeks hence will confront Reagan with a coup. Despite all the signals that it is coming, the president has not been prepared for it.