President Reagan stands at a troublesome crossroads as he enters the second year of a high-risk presidency made more difficult by the successes of Year One.

Unemployment is a decimal point away from its postwar high. The president's economic advisers are bitterly divided over tax and monetary issues. The National Security Council, its effectiveness as policy broker diminished by a year of conflict, is being rebuilt under a new national security affairs adviser. The vaunted unity of the top trio of presidential advisers has been shattered. And Reagan has shown himself to be embarrassingly unaware or misinformed as to the content of some of his basic policy decisions.

Unlike other recent chief executives, and Reagan during the early years of his California governorship, the president has no built-in alibi that will enable him to share policy failures with the legislative branch.

After years of deadlock between Congress and the White House, Reagan's political leadership and communicative skills let him break through and win decisive acceptance of his budget cuts and tax program. In doing so he created the conditions under which his economic programs can be judged on results rather than the conflicting claims of election-year rhetoric.

When Reagan took office a year ago yesterday, the conventional wisdom in Washington was that the all-powerful presidency was a casualty of congressional reaction to the Vietnam war and Watergate.

Reagan's well-conceived legislative victories made mincemeat of this view, demonstrating that a president capable of focusing on an issue and taking his case to the people still can prevail over Congress.

The president also demonstrated a presence, style and charm which won him personal admirers even among those who do not share his political opinions. When he went on vacation, which he did as often as he could, he did not pretend that he was studying briefing papers. Reagan's cheerful courage in the aftermath of an attempt on his life last March 30 gave the nation a glimpse of a natural man who used wit as a weapon in the face of danger.

As he faces a second year in which the easy answers of the first seem increasingly suspect, Reagan's personal qualities remain his prime political assets. Former president Richard M. Nixon has offered the opinion that Reagan may be "too nice to be president," but the niceness has worn well with voters.

Though Reagan's approval rating has dropped notably in every major public opinion survey, the president remains far more popular than his economic programs.

A New York Times-CBS News poll this week found Reagan with a 49 percent approval rating, although only 37 percent of the poll respondents said they were better off than they had been a year ago, a test Reagan asked voters to use in measuring President Carter's four-year performance.

Similarly, a Washington Post-ABC News poll last November gave Reagan a 53 percent approval rating, though only 45 percent of the respondents approved of the president's handling of the economy and only 34 percent of the way he has dealt with unemployment.

Reagan's poll ratings, even of overall support, are now slightly below Jimmy Carter's at the end of his first year in office. The Gallup Poll gives Reagan a December approval rating of 49 percent, compared with 51 percent for Carter, 56 for Nixon, 71 for John F. Kennedy and 61 for Dwight D. Eisenhower at similar points in their presidencies.

In the face of rising unemployment already far above the level Reagan termed "a depression" during the campaign, the White House political nightmare for 1982 is of a belated and not especially strong second-quarter recovery accompanied by the rapid return of high interest rates.

Round Two of this nightmare is a Democratic landslide in the November elections. Round Three is a Congress with such an overwhelming Democratic majority that Reagan becomes a lame duck in 1983.

Reagan, as usual, has a more optimistic vision of the future, and so do many voters. The Times poll found 60 percent of the voters agreeing that Reaganomics will work ultimately. This poll and others also found that a majority of Americans approve of Reagan's handling of foreign policy, and specifically of the way he has responded to the Polish crisis.

Reagan's national security operation is likely to be far more smoothly run under William P. Clark in 1982 than it was in 1981 under Richard V. Allen.

And Clark may help quiet the feuding between two Reagan Cabinet strong men, Secretary of State Alexander M. Haig Jr. and Secretary of Defense Caspar W. Weinberger. Haig has taken command at State, and administration officials say they believe that 1982 will herald the coming of genuine arms reductions talks and perhaps an international summit meeting.

"Don't pass off Reagan's commitment to arms reduction as campaign rhetoric," a high-level administration official said recently. On the whole, there seems to be more optimism within the administration over the president's foreign policy prospects than his domestic ones.

More disquieting than Reagan's performance or prospects on any specific issue is a growing suspicion that the president has only a passing acquaintance with some of the most important decisions of his administration.

It is a suspicion fueled by Reagan's performance at several of his recent news conferences and by casual remarks on other occasions.

Last Oct. 2, when Reagan announced a plan to upgrade U. S. strategic weaponry, a plan that was supposedly the product of long presidential deliberation, he said he was abandoning the Carter administration's proposal to deploy MX missiles in "racetrack" loops in the deserts of Nevada and Utah. Yet that deployment method had been dropped by Carter in April, 1980.

On Dec. 17, at a news conference, Reagan displayed ignorance of his Justice Department's efforts to overturn the Supreme Court's decision allowing companies and unions to agree even voluntarily on affirmative-action plans.

On Dec. 18 the president was asked on the Today show by Tom Brokaw whether he was prepared to restrict private or voluntary food shipments to Poland.

" . . . You know, this is a point that in all the haste of this thing coming that I don't think I've been present in any discussions of that," Reagan replied. "I would think, however, that that would be suspended just as our own government aid is. I would have to check and make sure that that is true."

Actually, a proposal was then being discussed in the administration to suspend government aid but allow private food shipments to continue. Reagan eventually accepted this. But when he was asked about this during an Oval Office photographic session four days after the Brokaw interview he was still not conversant with it and said it was "too early in the day" to comment because he hadn't attended any meetings yet.

This week, Reagan botched several questions at his news conference, misquoting economics statistics on which he had been briefed and erroneously remembering the nature of the "loophole" in the California abortion law he had signed as governor.

The overall impression Reagan has created steadily by such comments may be more troublesome for him in the long run than a series of well-publicized staff conflicts and foul-ups on such issues as the Equal Rights Amendment, the sale of advanced fighter planes to Taiwan and the question of tax exemptions for segregated schools.

Increasingly, Reagan appears uninformed on what is happening within his own administration. As he begins his second year he has raised anew one of the principal issues of his presidential candidacy.

Simply put, the question: is Reagan up to the job?