Taiwan could continue to buy arms from foreign countries after reunification with China, Deng Xiaoping, China's leader, has told a visitor from the United States.

In a two-hour meeting in June with Winston L. W. Yang, a professor of Chinese studies at Seton Hall University in South Orange, N.J., Mr. Deng, China's senior Deputy Prime Minister, outlined his views on reunification with Taiwan, raising several new points and refining Peking's known position on Taiwan autonomy after reunification with the mainland. Mr. Deng said he hoped his statements would temper any fears the Taiwan Government had on starting negotiations on reunification.

''We are reasonable, we are sensible and we are sincere,'' Mr. Yang quoted Mr. Deng as saying.

China has said Taiwan could retain its own armed forces after reunification but has balked at the suggestion that Taiwan could continue to buy arms abroad.

While Peking has said it would let Taiwan remain an autonomous region, Mr. Deng outlined six areas in which Taiwan would keep its independence after coming under control of the mainland, according to Mr. Yang. These areas were:

- Taiwan would control its own foreign affairs, issue its own passports and visas and develop its own economic relations with other countries.

- After reunification, China would send neither troops nor officials ''to take over, to manage, to supervise or to interfere with Taiwan's internal affairs,'' Mr. Deng was quoted as saying.

- Taiwan would retain its own Government, including its legislative and executive branches, and the laws of China would not apply to Taiwan.

- Taiwan would keep its own judicial system, which would not be subject to control or review by China's Supreme Court.

- China would not impose a Communist economic system on Taiwan.

- Taiwan would have its own flag, although it would not be the flag it uses now.