Fearing that their Khmer Rouge allies in Cambodia are facing hard times the Chinese have begun to show renewed signs of a significant future shift in favor of avowed neutralist Norodon Sihanouk.

After four months of estrangement in which Sihanouk took up residence in North Korea, the Chinese invited Sihanouk back to Peking as an honored guest at their Oct. 1 national day festivities and have encouraged him to continue to remain here. They have apparently curtailed efforts to persuade Sihanouk to support the Pol Pot-led Khmer Rouge whom Sihanouk describes as "murderers," while allowing the Cambodian prince to lead a life of luxury in a spacious compound.

By renewing their apparent ties with Sihanouk and remaining silent while he derides Pol Pot in long chats with Western journalists, the Chinese indicate how weak they feel their position in Cambodia has become. About 170,000 Vietnamese troops are now moving against the remnants of Pol Pot's forces in northwestern Cambodia.

Military analysts in Bangkok estimate that the Khmer Rouge army of Pol Pot, who was driven from Phnom Penh by Vietnamese forces in January, still has about 25,000 men and women under arms. The analysts do not expect the Khmer Rouge to defeat the numerically superior Vietnamese forces on the battlefield, but they say that Pol Pot's army appears capable of inflicting heavy casualties on the Vietnamese in the dry-season guerrilla fighting ahead.

Turning to Sihanouk would mean a special relationship with a man who still inspires loyalty among many surviving Cambodians, but who has only a tiny force in Cambodia and insists on a noncommunist future for any Sihanouk-led state.

"I am the Sihanouk card. First they deal Pol Pot, and if that doesn't work, then they deal Sihanouk," he said. One Western diplomat here said: "There is no question that the Chinese are now changing their mind about Sihanouk and preparing for Pol Pot's defeat." That message has been received by many Western governments, leading in part to the American, British and Japanese ambassadors calling on Sihanouk here and, in the view of some, encouraging the U.S. State Department to say Sihanouk "could have a constructive role to play" in a future Cambodia.

As the Chinese begin to show renewed interest in him, Sihanouk also has begun to shelve some of his more independent schemes that particularly irked the Chinese.

He had announced early plans to seek talks with the Vietnamese, Peking's arch-rival, on the Cambodian question. Now he dismissed such plans as fruitless. He promises that within two months he will be ready to support China's policy of all-out armed struggle against Hanoi's invasion force.

Sihanouk said he has sent the Chinese a message indicating that at the end of that two-month period, "I am ready to negotiate with them again."

The Chinese have dealt with Sihanouk for a long time. When he was overthrown by rightists in 1970, the Chinese invited him to live in Peking. The late Premier Chou En-lai seemed particularly interested in encouraging Sihanouk's government in exile.

The victory of Pol Pot in 1975 enticed Sihanouk to return to Phnom Penh, but there he became a virtual prisoner. When Pol Pot released him to return to Peking in January of this year, Chou was dead and Sihanouk was no longer so willing to deal with a Chinese government that had backed Pol Pot.

"The trouble with Sihanouk is that he's irrational and unpredictable," said one Chinese official, commenting on his government's often ambiguous attitude toward the prince. "We never know what he'll say from day to day."

Nonetheless, the Chinese have given Sihanouk full rein to organize a new "confederation of Khmer nationalists," which he said he plans to promote with trips to France, Japan, the United States and Australia beginning in November. Sihanouk said he planned to return to Pyongyang next spring in time for the birthday of his patron, Kim II Sung of North Korea, but indicated he might resume permanent residence in Peking if the Chinese conclude by then that Pol Pot's position is absolutely hopeless.

In the meantime, despite his love for tweaking his Chinese hosts and their Khmer Rouge proteges in conversations here with Western journalists, Sihanouk's jokes and exaggerations hide a keen sense of the diplomatic art. He has refrained from official requests to France and The United States for aid for his tiny guerilla force of 5,000 in Cambodia, knowing that the request would be refused and thus embarrass both the freindly Western governments and himself.

He also has refrained from announcing another government in excile, knowing the Chinese cannot support such an initiative unless Pol Pot is completely defeated. Instead, he waits patiently for the Chinese to accept him on his terms.

"I learned cooking by myself; I learned English by myself," he said at the luncheon prepared under his exact instructions. "I don't like to obey anybody; I like to be independent always . . . I love China. But it is like having two wives. When China has two wives, Pol Pot and Sihanouk, when China praises Pol Pot, Sihanouk is jealous.'