A high-level diplomatic mission from the Soviet Union has arrived here for the first political consultations with China in nearly three years, an official Chinese spokesman announced today.

Deputy Foreign Minister Leonid Ilyichev, who heads the Soviet delegation, will meet with Chinese officials for talks "on the question of Sino-Soviet relations," said the spokesman for China's Foreign Ministry.

Diplomats say the talks will be preliminary and exploratory, aimed at finding common ground for future cooperation in economic and cultural areas while leaving larger strategic issues unresolved.

Nevertheless, these discussions and a second round reportedly planned for later this year in Moscow are considered a significant step toward improving relations between the sparring Communist powers.

Chinese officials have ruled out any fundamental normalization of relations with Moscow while it maintains its aggressive military stance in Asia.

But Peking recently has displayed a new flexibility in dealing with its old arch-rival by expanding everything from trade to sports exchanges and now agreeing to high-level political talks, according to diplomats.

China and the Soviet Union have quarreled bitterly over strategic and ideological issues for 20 years, giving a triangular shape to the global power balance, with the United States at the third corner. Both Communist giants have turned at different times to Washington as a counterbalance against the other.

Until recently, China courted Washington as a strategic partner against what Peking called "the polar bear," and just a year ago it was considering U.S. offers to sell it weapons to strengthen its defenses along the contested Sino-Soviet border.

Frustration over U.S. arms sales to Taiwan coupled with persistent Soviet peace overtures have inspired a shift in Chinese foreign policy, giving it a more centrist position between the two superpowers.

With the Taiwan arms issue defused by a Sino-American communique issued in August, Peking lately has concentrated its fire on U.S. trade restrictions and bureaucratic delays in selling China such innocuous equipment as civilian computers.

"Loud thunder, little rain," complained a senior Chinese official, reflecting Peking's disappointment over what it says are unfulfilled American promises to trade freely with China.

Moscow has sought to exploit Peking's growing disillusionment with Washington as well as the Chinese government's open desire to stabilize its tense borders so it can concentrate on the task of modernizing its society.

After pressing for talks in vain, Moscow decided last month to dispatch Ilyichev, a top diplomat who headed the Russian negotiating team at the last Sino-Soviet political contacts in 1979.

Peking, which broke off the contacts after the Soviet invasion of Afghanistan in December 1979, initially balked at another Ilyichev visit, refusing to commit itself to talks or to sponsor him as an official guest, according to well-informed European diplomats.

China is believed to have shifted policy after Moscow agreed to open the agenda to all facets of bilateral relations, including Peking's three central complaints: heavy Soviet troop deployment along their border and in Outer Mongolia, Soviet military occupation of Afghanistan and its support for Vietnamese forces in Cambodia.

Diplomats said the Soviet willingness to discuss these topics is a concession to get talks started. Two months ago, Yu Hongliang, chief of China's Soviet desk, had traveled to Moscow to discuss these security concerns but found the Soviets unwilling to talk.

"The Soviets told him they couldn't discuss their relations with a Third World nation," said a diplomat. "They said if Peking wanted to discuss Afganistan problems it should take them up with the Afghan government."

It remains unclear whether Ilyichev has come as an official Chinese visitor or as a guest of the Soviet ambassador here, as originally planned. He is said to be staying at the massive Soviet Embassy in Peking, and diplomats say they know of no Chinese invitations.

China's Foreign Ministry refused to clarify the matter today, saying only that Ilyichev arrived "according to the decision made by the Chinese and Soviet sides through consultations."

According to the spokesman, Ilyichev will meet with Vice Foreign Minister Qian Qichen, head of the ministry's department of Soviet Union and East European affairs.

Diplomats said initial discussions are expected to be narrow in focus. China is known to be seeking scientific exchanges with Moscow while the Soviets are believed to be angling for a consular treaty to protect Russian nationals living in China.