The Palestine Liberation Organization's chief foreign affairs official left the Soviet Union today after preparing the way for a visit here next month by PLO Chairman Yasser Arafat, diplomats said.

Farouk Kaddoumi's official three-day visit was described by one western diplomat as a "significant" development in Soviet relations with Arafat, who had been criticized here for his agreement last year with Jordan to work jointly toward peace with Israel.

The Soviet news agency Tass reported that Kaddoumi was received today by Soviet Foreign Minister Eduard Shevardnadze. Middle East specialists said it was the first time in more than four years that a PLO official has been accorded such high-level treatment here.

Tass described the talks as having taken place in "a friendly and businesslike" atmosphere.

Kaddoumi's reception and the Arafat visit, timed for the 27th congress of the Soviet Communist Party, are signs that the Soviet Union has decided it is time to improve its relations with the PLO leadership.

Those relations deteriorated a year ago after Arafat reached an agreement with Jordan's King Hussein to move toward a settlement with Israel. The Soviet Union has opposed "separate" settlements in the Middle East, pushing instead for a peace conference in which it would take part.

After the Arafat-Hussein agreement, the Soviet press became more critical of Arafat, although it later shifted the focus of its attacks to Hussein.

At the same time, Moscow has defended Arafat's leadership of the PLO against attacks by Syria, and in meetings with Syrian officials, Soviet spokesmen have publicly warned against interference in Palestinian affairs. Today, Tass said Kaddoumi and Shevardnadze agreed on the danger of "separate capitulatory deals with Israel."

Arafat's planned visit to Moscow -- which will be his first since the funeral of Soviet leader Yuri Andropov in February 1984 -- could be taken as a sign that he is backing away from the agreement with Hussein, one western diplomat said. It also could serve to solidify Arafat's own position as Jordan and Syria show signs of reaching an accommodation, the diplomat said.

One Middle East specialist attached significance to the Soviets' move to put relations with the PLO on a higher diplomatic footing. Since Soviet leader Mikhail Gorbachev came to power, Soviet diplomacy has put greater stress on establishing and maintaining contacts, particularly in the Middle East.