Since 1970s, South China Sea has been a controversial area because of its important strategic location and abundant resources. China, Taiwan (Republic of China), the Philippines, Vietnam, Malaysia, Indonesia and Brunei had claimed sovereignty of island and maritime territory in the South China Sea. For China, the importance of South China Sea is obvious. China insists on its historical rights over the South China Sea and firmly opposes to the South China Sea arbitration case unilaterally initiated by the Philippines. Before 2010, China kept a low profile based on the principle of shelving differences and seeking joint development. It not only participated in signing with ASEAN but also cooperated with other dispute parties in the field of research and resource development. But since 2010 China has taken administrative, military and other strong measures to strengthen its existence in the South China Sea, resulting tensions over the area. Why China changed its policy towards the South China Sea after 2010? In order to answer this question, this study intends to use Beasley's foreign policy-making factors theory as framework to analyze the factors which affected China's South China Sea policy from internal(domestic) and external(international) perspectives. My research suggests that many factors are associated with China's assertive South China Sea policy. First, the rise of China's economical, energy security situation, and the rise of nationalistic public opinion are the objective factors. Then, Leader, local government, PLA and state-owned energy companies had effect on the policy-making towards the South China Sea as domestic actors. Internationally, the behaviors of some aggressive dispute parties and some countries outside this area also encouraged China to change its South China Sea policy from moderatism to tough line. These factors played roles in China's South China Sea policy in a dynamic and progressive process. Besides, there are interactions among them. It is difficult to resolve the dispute over the South China Sea properly in a short time, but recent international changes offer the opportunity. Currently, the most important thing is that the dispute parties don't expand and complicate the dispute anymore and start making efforts to resolve the dispute through peaceful negotiations. Only through this way the peace and prosperity in the South China Sea can be maintained.