China today protested U.S. plans to sell Taiwan $60 million worth of military spare parts, but backed off from threats to downgrade relations over the weapons supplies.

The protest, coming a day after the Reagan administration formally notified Congress of the sale, was seen as rather mild in light of recent Chinese warnings and as a sign of Peking's desire to defuse the issue for the time being.

A spokesman for the Chinese Foreign Ministry charged that the spare parts deal infringes on the sovereignty of China, which considers Taiwan a breakaway island that could be brought back to the fold if forced to deal with the mainland without American arms.

Peking's anger was assuaged, however, by U.S. pledges to limit the pending sale to spare parts and to freeze all new weapons transfers to Taiwan while Chinese and American officials continue to discuss the problem through their embassies, according to the spokesman.

He said Peking "took note" of the American explanation that the spare parts at issue--which include brake shoes and spark plugs for F5E fighter aircraft--were promised to Taiwan before Sino-American talks on the subject began in November.

Should Washington renege on its assurances and "continue to disregard China's sovereignty," said the protest note delivered to U.S. Ambassador Arthur Hummel, "it must be held responsible for all the consequences arising therefrom."

China has been demanding guarantees from the United States to phase out and set a deadline for military sales to Taiwan, which was an American ally for 30 years before Washington shifted its recognition to Peking in 1979.

Chinese officials have declared that the sale of a single rifle represents an interference in China's domestic affairs and violates the 1979 agreement in which Washington acknowledged that Taiwan is part of China.

As negotiations deadlocked over Chinese demands, Peking became so truculent in its warnings that foreign diplomats widely believed it would have to downgrade relations with the United States just to preserve its credibility.

Last month, Vice Chairman Deng Xiaoping was quoted in an official magazine as saying that the U.S. handling of the Taiwan arms matter is "unacceptable" and that Peking is "well-prepared" for a downgrading if military sales continue.

"On this question, we have little room for maneuver," said Deng, who is China's foremost political leader.

Compared to Deng's hard line, today's protest--which the Chinese Foreign Ministry said was "strong"--was viewed by diplomats as surprisingly cautious and accommodating. One West European envoy called it "Peking's cop-out."

Other analysts believe China pulled back from the brink because the spare parts to be sold Taiwan add nothing new to its arsenal. The real test will come if the Reagan administration goes ahead with plans to sell Taiwan new F5E jet fighters.

"This is a question of national dignity," said an East European diplomat. "But even dignity comes in degrees."

Analysts agreed that the moderation expressed by China today indicates that it will not easily give up the strategic and economic benefits of good relations with the United States.

Although Moscow is trying to capitalize on Sino-American friction by offering to normalize relations with Peking, Chinese officials have vowed never to play the "Russian card." Soviet troops stationed along China's northern border and in Afghanistan are seen as a direct security threat.

Chinese officials believe that an entente with the United States, Japan and Western Europe is the best deterrence against the Soviets.

Economically, China gets its largest grain imports from America and is counting on U.S. oil companies to lead in the exploitation of offshore oil.

The United States is seen as the only country large enough and wealthy enough to absorb the thousands of Chinese scientists and technicians who want to study abroad to aid China's modernization campaign.

Although Peking warns that the sale of arms to Taiwan could poison political relations, officials express the hope that the burgeoning economic ties between the two nations could continue.

But Western diplomats say a downgrading of relations could undermine American investor confidence in a less friendly China.

While lowering its voice today, Peking is expected to keep up the pressure for a cutoff in arms sales to Taiwan while Congress considers the administration proposal for spare parts. Routine approval is expected.

Last year, China downgraded relations with the Netherlands to the level of charge d'affaires after the Dutch government decided to sell two submarines to Taiwan, but the final recall of ambassadors only came after the Dutch parliament approved the sale.