Taiwan quickly rejected China's offer earlier today to include Nationalist Chinese officials in the leadership of a reunited nation, although diplomats pointed out that the proposal goes far toward meeting the previously unpublicized demands of the Taiwanese.

In Taipei, the Nationalist Chinese government said Peking's latest offer was another propaganda stunt aimed at subjugating free people under communist rule. The proposal "contains nothing new," spokesman James Soong said. "The only way to achieve unification is to abandon the communist system."

Diplomats indicated that the offer-- which includes a call for unconditional talks -- was the most conciliatory gesture yet by China, reflecting a growing sense of urgency to settle the long civil war before new foreign or domestic developments drive the two sides further apart.

The Nationalist conditions, which have been expressed privately, are believed to include guarantees of some role for Nationalists after reunification, equality of the two parties in negotiations and the withdrawal of Peking's demand that the Nationalist flag and anthem be abandoned.

The offer put forth this morning by China's parliamentary chairman and Communist Party vice chairman, Ye Jianying, pledges to allow Taipei officials to "take up posts of leadership in national political bodies and participate in running the state" after the island rejoins the mainland.

Ye's statement failed to explain how Nationalists could participate in the communist government, but diplomats here have speculated that officials in Taiwan, especially those specializing in economic development, could be placed in technical posts.

A second Nationalist demand appeared to have been answered in today's proposal when Peking offered to hold talks with the Nationalist Party "on a reciprocal basis." Again, terms went undefined, but diplomats assume "reciprocal" to mean equal.

Finally, the Ye statement called for talks without setting the preconditions cited in earlier overtures, such as the demand for Taiwan to give up its flag and anthem. For the first time, there was no mention of that requirement.

As an unconditional offer, Peking suggested that each side send representatives to "meet for an exhaustive exchange of views."

The Chinese started their diplomatic offensive in January 1979, calling for trade, travel and military negotiations with the estranged island government. The latest offer, diplomats said, reflects the privately stated belief that the time is ripe to inaugurate talks because both sides are headed by powerful men -- Chiang Ching-kuo in Taiwan and Deng Xiaoping in China -- with enough influence to make the compromises necessary for reunification after 32 years.

Communist leaders are known to regard Chiang as a patriot who shares their goal of a united China, albeit on Nationalist Chinese terms. But he is in his seventies and suffering from diabetes, leaving little time to achieve his lifetime hope. When he dies, Communists fear there will be no politician strong enough to negotiate for the island government.

At 76, Deng also has numbered days in power. He is said to be under considerable pressure from old-line Communists to justify his peaceful overtures to Nationalist enemies. Although Deng possesses the prestige to pursue the policy toward Taiwan, it is unclear whether his successors would without positive results in the interim.

After nearly three years, Deng has little to show for his Taiwan initiatives. Despite Peking's promises -- reiterated today -- that Taiwan can maintain its capitalistic economic and social system as well as its armed forces if it agrees to reunite with the mainland, the refrain from the breakaway island has continued to be, "No, never."

Deng and his moderate allies also may have fashioned today's proposal to head off what the Chinese believe is an impending U.S. decision to sell new sophisticated arms to Taipei, a development Peking strongly opposes on the ground that new weapons would only harden the Nationalist resolve to avoid talks.

The latest mainland concessions, according to Asian diplomats, may be designed to demonstrate China's reasonableness and commitment to peaceful reunification in the hope that the Reagan administration, which is now considering the arms sales, will do nothing to upset the blueprint.

By emphasizing conciliation, Peking also undercuts Taipei's position at home and abroad, thus increasing pressure on the Nationalists to sit down for negotiations, a Western diplomat said. By offering to give Nationalists positions in a reunited China, Peking made it even harder for Taipei to resist talks.