China's diplomatic and security thinking have undergone a fundamental change since its adoption of reform and opening policy in the 1980s. This article first analyzes the causes and practices of China's multilateral diplomacy after the 1980s. It is followed by an examination of the concept of multilateralism in international relations theories. Finally, this paper applies the concept of multilateralism to China's practice of multilateral diplomacy in its neighboring areas, and then compares the concept and China's practice to see how this concept has been practiced in China. The major finding of this paper is that China has developed a flexible practice of multilateralism with Chinese characteristics, through which China can consolidate its sovereignty. This is quite contrary to the traditional western thought of multilateralism, and it is an important area worthy of further investigation.