Abstract: America has been controlling the strategic chokepoint of South China Sea, and the extension of China’s strategic interest is threatening America’s control status in South China Sea. In this context America put forward the strategy of returning to Asia-Pacific in order to strengthen the control of South China sea and prevent the challenge from China and maintain its hegemony. This article studies why did America return to Asia-Pacific from the perspective of South China Sea’s marine strategic access. The research found that America intervened in the South China Sea’s dispute is based on internal and external reasons. The internal motivation is that America’s grand strategy determines it must control the marine strategic access and it wants to turn the South China Sea into its global 17th marine strategic access. The external reason is that China’s activities in the South China Sea squeezed America’s space of marine strategic access. China should plan the strategic responses on the premises of not influencing national grand strategy and marine strategy.