A senior State Department official arrived here today to reopen, at a key time, a high-level dialogue with China on strategic issues thought to include Indochina, Afghanistan and the limits of Sino-U.S. military coordination.

Michael Armacost, undersecretary of state for political affairs, said he expects to discuss a "broad range of global and regional issues" with Chinese officials, including Vice Premier Yao Yilin and Foreign Minister Wu Xueqian on Monday and Tuesday.

The four-day visit comes as Sino-U.S. relations are moving forward on a number of fronts. At the same time, Peking has sought to improve relations with the Soviet Union following Mikhail Gorbachev's assumption of power there last week.

Armacost said he, too, expected to discuss the Soviet transition. Another likely issue is possible U.S. assistance in the modernization of China's military forces.

According to officials and defense experts, there is a need to clarify how far the United States is prepared to go in its efforts.

According to several of them, the U.S. Navy has moved faster and farther toward agreeing to help modernize destroyers in the Chinese fleet than some State Department officials would like. Navy Secretary John F. Lehman Jr. visited China last August.

There are also divisions reported within the Defense Department as to how fast and how far the United States ought to move in defense cooperation here. One U.S. defense specialist argued in Washington recently that there are at least four schools of thought within the Pentagon on this issue, ranging from the standard official view that the countries have parallel strategic interests to those contending that China is a strategic liability.

"In Lehman's view, it's full speed ahead," said this specialist, who asked not to be identified. "He did a lot on his own initiative. He got way ahead of the pack."

One argument apparently being made inside the U.S. government for expanding the defense relationship is that it gives China's top military officers more of a stake in the nation's relations with the United States and in China's modernization process.

Until recently, at least, the Chinese leadership has given the military a relatively low priority in this process. Some military officers are believed to be less than enthusiastic about China's opening to the West.

Outside the government, there is some concern among American scholars on China that a major Navy assistance program will increase the pressure from supporters of Taiwan in Washington to sell more arms to Taiwan.

Some scholars contend that the Reagan administration must coordinate its policy or be manipulated by other powers in a strategic triangle of China, the United States and the Soviet Union.

The U.S. reaction to the recent warming of Sino-Soviet relations appears to have been calm, but a specialist said there is concern in some administration circles "asking just how far it's going to go . . . . Some of the antennae are quivering now."

In the past, senior Chinese officials have said Sino-Soviet relations could not improve until the Soviet Union acted to remove its "threat" to China. They defined this threat as the presence of hundreds of thousands of Soviet troops along the border, the Soviet invasion of Afghanistan and Soviet support for Vietnam's actions in Cambodia.

One of Armacost's aims presumably will be to determine whether the United States and China still share views on these issues.

With U.S.-Chinese relations moving forward rapidly in a number of spheres, it may be easier to talk about coordinating policies than actually to do so. But in the past, trips to China by high-ranking officials have had the effect of propelling both bureaucracies to set priorities.

Cabinet-level visits to China in 1983, for example, set the stage for breakthroughs in high-technology sales.