After persistent prodding by his secretary of state, President Reagan has changed his mind and given preliminary approval for the drafting of a "State of the World" speech that would be the first full statement of his year-old administration's foreign policies.

This is not to say that Reagan, who has often expressed an aversion to such things, has irrevocably committed himself to delivering a sweeping foreign policy address, presidential advisers stress. But he has now declared that he is interested in seeing what a complete exposition of his administration's policies, as drafted by his top aides, would say.

"The idea of a State of the World speech has been put before the president and he has indicated an interest in it," said one senior presidential adviser. "I know that State feels it is scheduled. But the White House does not feel it is signed off on it yet. No date has been set. But the president is interested in seeing what such a speech would say."

If he gives it, it will be in February, after the State of the Union message on Jan. 26. According to several administration officials, it probably will bear a strong resemblance to the speech draft that was originally produced by the State Department--and scrapped by the White House--during preparations for the president's only other major foreign policy address: his well-received Nov. 18 speech proposing the reduction of nuclear arms based in Europe.

The State Department had sent the White House an ambitious draft text of that November speech that was far more than just a European arms address, according to informed sources. It offered a tour d'horizon, containing sections outlining U.S. policy for each global region.

But the president and his White House advisers felt that would have diffused the major message he wanted to send, and perhaps provoked a series of intramural administration policy skirmishes in the process.

The president opted instead for a speech focused solely on reducing nuclar arms in Europe, built around the so-called "zero option" proposal that had been championed in the inner circle by Defense Secretary Caspar W. Weinberger and opposed by Secretary of State Alexander M. Haig Jr.

The favorable reception that greeted Reagan's speech in Europe (where it was beamed live via satellite) and in the United States may have gone a long way, Reagan officials say, toward softening his longstanding aversion to suggestions that he deliver an address detailing his foreign policy views.

He has often scoffed publicly at the notion that it is advisable to let others know just what global policies he intended to pursue. As recently as last December, in an interview with newspaper and television commentator Ben Wattenberg, Reagan maintained that his administration had a grand design for its foreign policy, even though it has never been articulated publicly.

". . . the only difference is I didn't announce it letter by letter and put it on the front page of the papers," Reagan said. "First of all, I think that's kind of foolish to do, because much of a foreign policy must be concerned with rather delicate negotiations, quiet diplomacy, getting things done, not by challenging someone, and then putting his back up in the other country because politically he'll look like a weakling if he agrees with you."

Haig, among others, has long urged that Reagan deliver a comprehensive address on foreign policy, administration officials say. And last month (just about the time Reagan was telling interviewer Wattenberg that such things are "kind of foolish"), the president's advisers presented him with a Haig-backed proposal that he deliver a "State of the World" message.

It is important, these advisers argued, that America's allies and adversaries understand just what the Reagan administration intends to do and what it will be asking of the other nations of the world. And to the surprise of some of the officials, this time the proposal struck a receptive presidential chord. Reagan told his advisers to go ahead with the outlining and planning for such a speech.

The idea of a "State of the World" presidential message is not new; it was begun by Haig's former employer, President Nixon, at the urging of Haig's former boss, Henry A. Kissinger, when he was national security adviser. Ever since, presidents have delivered to Congress lengthy messages on international policy, but usually in written form only.

Reagan is interested in delivering his message in person, not just in writing, according to one White House official.

But several administration officials voiced concern that the process of shaping a "State of the World" message may touch off a new round of intramural policy struggles, as the Reagan officials seek to put into writing what they have already been putting into practice.

That, they noted, is what happened in the fashioning of the only other major foreign policy speech that the president has made, the November speech on European arms, which Weinberger had strongly pushed.

Haig opposed it as too blunt a statement, leaving no room for a fallback position in negotiations. State's draft contained only an ambiguous reference to the possibility of a "zero option."

"In the end, what the president finally proposed was just what we had fought and bled for," a senior Defense Department official said the other day. "But I'm not sure I want to go through that again on policy decisions on the Middle East and every other area."