The Reagan administration, alarmed at deteriorating conditions in Mexico, has begun reviewing what officials acknowledge is a fragmented U.S. policy in an attempt to stave off serious damage to both countries.

As the United States' third largest trading partner with $33 billion in commerce last year, Mexico has cultural, legal, social, labor and economic ties across the Rio Grande that lock the two nations' fates together. Some officials, however, believe the ties are generally ignored except in times of crisis.

Mexican issues -- including drug enforcement, immigration and the country's huge debt -- are handled disparately by different U.S. agencies sometimes working at cross purposes, according to some officials.

Consequently, Treasury Secretary James A. Baker III, national security adviser John M. Poindexter, law enforcement agency heads and other senior officials met last week to launch the coordination effort.

"The overall policy may be so broadly stated that it's not a policy at all," one key State Department official said. "Right now, Mexico has been discovered again up here. The question is how long will it last."

State Department and White House officials worry that the latest furor over conflicting administration statements on drug-related corruption in Mexico has exposed a lack of coordination in U.S. policy and diverted Mexican attention from urgently needed internal reforms.

In the U.S. view, Mexico's staggering $97 billion foreign debt, its plummeting income, soaring inflation and unemployment problems are directly related to the official corruption, growing political unrest, the rising flood of illegal immigrants across the U.S. border and cultivation of marijuana and heroin poppies by peasants desperate for income.

U.S. policymakers pushing for "structural reform" in Mexico are frustrated by their new inability to offer substantial financial assistance or trade incentives because of U.S. budget restrictions.

The latest upsurge in official Washington attention coincides with a key provincial election campaign in northern Mexico and with the month-long World Cup soccer tournament, which has brought the kind of international media presence and influx of foreigners to Mexico that sparked riots and demonstrations by restive elements in the past. The United States has provided Mexico with FBI advisers on security measures, and tensions are expected to be high until the election in the state of Chihuahua is over in July.

The review also comes as conservatives led by Sen. Jesse Helms (R-N.C.), are pushing for a harder line against Mexico, which they contend is undermining U.S. policy in Cen- tral America and turning a blind eye to U.S. security needs along the 1,942-mile U.S.-Mexico border.

Ironically, the lack of any Marxist insur- gency in Mexico has contributed to the lack of official attention from a U.S. administration focused on the so-called Reagan Doctrine of combating communist insurgencies in the Third World.

There is no disagreement within the administration over the existence of drug-related corruption in Mexico, but there was consternation when U.S. Customs Commissioner William von Raab went public with U.S. concern. He told a Senate Foreign Relations subcommittee that there was "massive" official corruption "up and down the ladder" of the Mexican government. He added that the governor of Sonora province owned four ranches "believed to grow opium and marijuana."

Attorney General Edwin Meese III called Mexican officials a few days later to say that von Raab's remarks "do not reflect the views of the president, the U.S. government or the Department of Justice," and the U.S. Embassy in Mexico admitted there was no evidence against the governor of Sonora and apologized to him.

Baker later put it more clearly: The charges "do not represent administration policy, even though they may be true," he told a group of reporters.

Secretary of State George P. Shultz also wrote a conciliatory letter to the Mexican government stressing the U.S. desire for continued good relations. But yesterday President Reagan again complicated the issue. White House spokesman Edward P. Djerejian said Reagan "has complete confidence in Mr. von Raab."

Asked how that squared with Meese's remarks, a State Department official had no comment.

In a statement after meeting with members of the Mexican legislature here for a Colorado session of the Mexican-U.S. Interparliamentary Union, Reagan said good will between the neighboring nations "should never be taken for granted on either side of the border." Friction "is to be expected between friends," he said, and in efforts to support fledgling democracies in the region, "the policies of Mexico and the U.S. need not be identical, but should not, on the other hand, work at cross-purposes."

That was taken as a reference to Mexican pressures for a Contadora regional peace treaty in Central America, despite U.S. reservations that a treaty will be unacceptable unless it leads to a democratic government in Nicaragua.

A Mexican diplomat said the 25 legislators were pleased with the meeting and saw it and Shultz's letter as gestures of good will. "We have never denied there is corruption in Mexico, but our worst problem is economic, not drugs," he said. "A [U.S. policy] review is a good idea because these contradictions are crazy . . . . How can we deal with things when every agency says something different?"