The Reagan administration is preparing to announce the resumption of full diplomatic relations with the military government of Bolivia, ending a 15-month absence of a U.S. ambassador in the country and continuing the administration's pattern of improving relations with Latin America's authoritarian rulers.

U.S. officials here said that the decision to send a new ambassador to La Paz was made after the two-month-old government of Gen. Celso Torrelio Villa pledged to move Bolivia toward democracy and curb its massive illegal cocaine trade with the United States, which reportedly involved high officials of the Army and the fallen government of Gen. Luis Garcia Meza.

The new U.S. ambassador is expected to be Edwin Corr, 47, a veteran diplomat who recently completed a term as ambassador to Peru and is a former assistant secretary for international narcotics affairs. Corr's name was reportedly submitted to the Torrelio government last week.

The Carter administration removed ambassador Marvin Weissman and suspended aid to Bolivia in July 1980 after Garcia Meza seized power from a civilian government in a violent coup. Since then, the United States had refrained from appointing a new envoy because of the drug trade and human rights violations by Bolivia.

The decision by the Reagan administration to reestablish full relations reflects a perception by U.S. officials that the government of Torrelio will correct the worst abuses of Garcia, whose stern and scandal-ridden rule made it the target of foreign critics and led to diplomatic sanctions by a number of Latin American and West European governments.

The Carter administration had distanced itself from authoritarian governments in the region because of their human rights violations. In recent months, the United States has suspended the Carter policy of voting against international bank loans to four other dictatorships surrounding Bolivia -- Chile, Argentina, Paraguay, and Uruguay -- and persuaded Congress to lift bans on arms sales to Chile and Argentina. Argentina has been a major supporter of the recent Bolivian military governments.

U.S. officials said this week that the administration had consulted with Latin American and Western European governments prior to deciding to name the new Bolivian ambassador, and expected that the U.S. initiative would be followed by similar moves in Western Europe and and possibly by such Latin American governments as Venezuela, which also withdrew its ambassador in 1980 after unsuccessfully trying to arrange the continuance of Bolivia's then-civilian government. Ecuador also currently has no ambassador serving in La Paz.

One reason for taking the action now, officials said, was to support what as seen as the relative progressiveness of Torrelio and discourage coup-plotting by military factions still loyal to Garcia or suspected of ties to the drug trade. Torrelio's new government is reported to be far from secure in a country that has seen almost 200 governments come and go in the last 150 years.

In meetings with State Department officials last month, Torrelio reportedly pledged to establish a timetable for returning Bolivia to democracy in three years, to abolish paramilitary squads, relax restrictions on the press, take economic measures including reduction of the foreign debt, and reform a state antinarcotics operation that had been allowed to disintegrate under Garcia.

Bolivia has also responded to pressure by the State Department and congressional committees by arranging the surrender in Miami of two narcotics traffickers who had been indicted by U.S. grand juries. "The drug-smuggling problem is still gigantic," one official familiar with the negotiations said. "But at least the government appears ready to take some steps to improve it."