Since 2010, China has carried out large-scale reclamation projects on the South Island Reef, and completed the militarization of the South Island Reef in 2017 to achieve full coverage of the South China Sea by military forces. In order to counter China's expansion of military power in the South China Sea, the United States actively implemented the counter-measure thought the right to freedom of navigation and strengthening the military joint exercises with the Asia-Pacific allies. This article will analyze the strategic thinking of China and the United States in the South China Sea in recent years from the perspective of Offensive Realism and explore China's military construction projects in the South China Sea, the US response and the military competition of China and the United States in the South China Sea in the future.