Poland's Supreme Court today reduced the prison terms of two leaders of the banned Solidarity trade union, but the measure fell short of expectations by western governments with which Poland is seeking improved relations.

A three-judge panel shortened the term of Adam Michnik, a historian and essayist, by six months to 2 1/2 years and reduced the sentence of Bogdan Lis, a former leader of the Solidarity union in Gdansk, by six months to two years.

The two men were sentenced last June together with Wladyslaw Frasyniuk, a Solidarity leader from Wroclaw, on charges of participating in an illegal organization and attempting to provoke arrest by planning a 15-minute strike. The strike was never held.

Today's ruling, which came after two days of hearings earlier this week, rejected appeals by defense lawyers that the men be retried because of procedural errors and irregularities in the original trial. The court also refused to reduce the 3 1/2-year sentence of Frasyniuk on the grounds that he was a repeat offender.

The modest sentence reductions for Michnik and Lis disappointed Solidarity leaders and western diplomats who have called on the government of Gen. Wojciech Jaruzelski to release the men, the most prominent of the 220 political prisoners now being held.

Solidarity leader Lech Walesa, who last week praised the dropping of his own trial on slander charges as "a sign of hope," said the Supreme Court's ruling today was "another disappointment" and "proof that the authorities are going to continue the policy of repressions and ignoring society's expectations."

Diplomats here said the continued imprisonment of the Solidarity leaders would undercut Jaruzelski's recent efforts to improve relations with western governments and win badly needed new credits for Poland's economy. Since late last year, the government has taken several steps to ease tensions with the West. Poland's financial planners have argued that the country must break a four-year drought of western loans in order to rebuild its economy and meet payments on the $30 billion it owes abroad.

In recent weeks, the government efforts at accommodation appear to have been channeled into negotiations with Poland's powerful Roman Catholic Church. Diplomatic sources said the dropping of slander charges against Walesa was negotiated in detail by church and government officials and that cases of other political prisoners have been taken up as well.

Government officials are seeking to arrange a trip by Jaruzelski to Rome that would include a meeting with Pope John Paul II, diplomatic sources said.

The trip, which would be the second by Jaruzelski to a West European country, has been connected to the government's agreement to a visit to Poland by the pope, a native Pole, in 1987, the sources said.

Finally, church authorities have received indications that Jaruzelski's government may be close to taking a major step to satisfy both the church and western governments by granting approval for a long-stalled church project to create a foundation chanelling funds to private farmers. The fund has received strong endorsements and modest donations from the Reagan administration and West European governments.

While these incipient developments have raised hopes of a new government liberalization in pursuit of church and western financial support, the modest action on the cases of Michnik, Lis and Frasyniuk appeared in line with past government concessions labeled as indecisive and unsubstantial by many western diplomats.

Much of the western dissatisfaction has focused on political prisoners, most of whom are Solidarity supporters previously active in underground workers' movements and clandestine publishing. Last year, Jaruzelski said while on a visit to the United States that the government might decree an amnesty freeing these detainees.

Eventually, however, authorities settled on a far more limited "humanitarian initiative" that only partially emptied jails. The number of political prisoners, which has continued to rise with new arrests in recent weeks, now is only slightly lower than it was when Jaruzelski first revealed the measure last October.

Frasyniuk and Lis, who both worked in Solidarity's underground temporary coordinating commission, were released from jail in 1984, but both spent only a few weeks in freedom before they were jailed again.