China has scorned South Korea's efforts to improve relations as "one-sided wishful thinking," according to an apparently official Peking document acquired by a human rights organization in the United States.

The document, a May 15 letter from the Chinese Foreign Ministry to the North Korean Embassy in Peking, attempts to diminish the significance of an unprecedented May visit to South Korea by Chinese officials seeking the return of a hijacked Chinese airliner.

Until the visit by Chinese aviation director Shen Tu, the Communist government in Peking has spurned any official contact with the anticommunist South Korean government in Seoul. South Korean officials portrayed the visit as a possible first step toward normal relations, much to the apparent distress of their adversaries in Communist North Korea.

In the letter, apparently designed to ease such fears, the Chinese said, "As for South Korea using this opportunity to demand the establishment of a formal channel of communication with China, all such messages are nothing but a South Korean monologue, reflecting its one-sided wishful thinking and have nothing to do with China's firm stance."

China Spring, a New York-based group of Americans and recent Chinese immigrants promoting human rights and democracy in China, gave a copy of the letter to The Washington Post. A spokeswoman for the group said that a Chinese Foreign Ministry official known to group members had provided the document, which had been hand-carried out of China through Hong Kong.

The spokeswoman said her group feared that the South Korean government might send the six Chinese hijackers it still holds back to China or punish them harshly in order to please Peking. The letter, she said, should convince Seoul that it has no chance of improving relations with China. A statement by the group said the hijackers acted "to escape the totalitarian rule of the existing regime . . . . The human rights of these hijackers should not become part of any political deal between two countries."

The letter noted Chinese Foreign Minister Wu Xueqian's scheduled May 20 trip to North Korea and repeated China's oft-stated position that the United States had caused the division of the Korean Peninsula and should remove its troops from South Korea.