Although today's arrival of Secretary of State Alexander M. Haig Jr. will turn China's attention to foreign policy matters, the three-day visit also is expected to have broad implications for the still-skaky domestic political course set by China's preeminent leader Deng Xiaoping.

The Sino-American normalization treaty that Deng helped fashion in 1978 not only fixed China's global strategy, but it also became the cornerstone of a controversial domestic policy featuring the gradual relaxation of social and economic controls and the pursuit of Western technology to help modernize China'ds creaky economy.

Three years later, Deng, 76, who heads a team of veteran pragmatists running China, is seeking reassurance from Haig that the Reagan administration will do nothing to shake loose that cornerstone by suddenly upgrading relations with Taiwan or dramatically increasing arms sales to the island.

The political stakes are high for Deng, who is believed to fear that any American rebuff on the touchy Taiwan issue could be seized upon by elements within the Communist Party that oppose his plans to reform the nation's institutions and replace the old guard with a new team of pragmatic politicians.

Diplomats here believe Deng needs evidence of a successful visit by Haig to assure a successful outcome of the upcoming plenary session of the party's Central Committee. The session is expected to mildy rebuke the late chairman Mao Tse-tune while elevating Deng's protege, Hu Yaobang, to the top party post.

The plenum has been delayed for months while Deng moved to appease leftists in the party and military who, according to diplomatic sources, began actively opposing him last year both on his reform program and on a whole host of issues connected to Mao's legacy. The leftists, many of whom benefitted from Maosism, are said to have resisted the depth of de-Maoization proposed by China's new leaders, the harsh treatment accorded to Mao's wife during the "Gang of Four" trial and the bald move to force out Mao's handpicked successor, Hua Guofeng.

As he moved to quell complaints of the left, Deng encountered resistance from the right of the party, a group of intellectuals and party regulars who bitterly suffered during the Cultural Revolution that Mao inspired. They demanded a thorough criticism of Mao and a purge of current public figures whose careers fluorished during the chaotic decade.

Although Deng managed to contain the leftist and rightist opposition while maintaining the basic lines of his reform policy, diplomatic and Chinese sources said that a setback on the Taiwan issue, which is considered the most burning national sovereignty question, could serve as a rallying point for his political enemies.

"Deng is caught in a crossfire from those on the left who say he's gone too far in downgrading Mao and implementing reforms and those on the right who say he hasn't gone far enough," said one China specialist.

That explains in part why the last week leading up to Haig's arrival has seen increasingly strident official statements and press commentaries condemning U.S. arms sales to Taiwan as an interference in China's domestic affairs and a violation of the treaty restoring Sino-American diplomatic relations.

The criticism along with Peking's warning of the "destructive effect" of contuined arms sales is apparently intended to set out China's maximum bargaining position for the Haig talks, according to diplomats. But it also is seen as an effort by Deng to show his politial enemies that he will not acquiesce on this important matter of national honor.

During the negotiations that led to normalization of Sino-American relations, the United States agreed to recognize Peking as the sole legal government of China and Taiwan as a part of China. In exchange, the Chinese emphasized their new policy of trying to reunify Taiwan by gradual peaceful means.

The only issue that remained unresolved at the time, according to participants in the negotiations, was that of U.S. arms sales to Taiwan. Both sides agreed to disagree for the time being, believing that the question could be better handled in the more relaxed atmosphere of normalization.

While the Chinese have always opposed the U.S. decision to continue arms sales to Taiwan after normalization, they began getting more anxious after Reagan promised during his campaign to upgrade relations with Taiwan and consider the Nationalist Chinese request for an improved fighter plane.

The longer Taiwan rejects the peaceful initiatives of the mainland's "smile diplomacy," according to Chinese sources, the harder it becomes for Deng and fellow moderates to stave off the staunch anti-Nationalists on the Chinese Politburo who argue that the only way to deal with their old enemy is through use of force.

In Politburo debates on foreign policy, analysts said, the continued arms sales to Taiwan raises questions about China's policy of associating with the United States, Japan and Western Europe in a highly confrontational posture toward the Soviet Union when Washington refuses to cut off arms to Taiwan.

That does not necessarily mean that China would seek rapproachment with Moscow if Sino-American relations faltered on the Taiwan arms sales issue, Chinese sources cautioned. It could mean, however, that China would slow down the pace of cooperation with the United States in an anti-Soviet coalition.