The U.S. army holds more than thousands of military exercises in Asia-Pacific region each year. The U.S. army keeps its military cooperation with allies and partners and maintains military hegemony by the opportunity of multilateral joint military exercises. Along with the growth of China's comprehensive national strength, the foreign policy has turned into active and positive. PLA strives to participate in various types of joint military exercises in Asia-Pacific region. PLA enhances its military capability and regional influence, and establishes military-friendly cooperative relations through multilateral joint military exercises. This paper analyzes the objectives, the content and the future development of the U.S. and China's participation in the multilateral joint military exercises in Asia-Pacific region from the views of Defensive Realism and Offensive Realism.