Abstract: The 19th National Congress of the Communist Party of China has opened a new chapter for China to enter a new era of socialism with Chinese characteristics, and has put forward a new expression of the transformation of major social contradiction with great historical adaptability, time tension and overall importance. Different from western welfare countries, under the background of the transformation of main social contradiction, the development of China's social security system is not about the issue of the system itself, but the issue of how the system construction adapts to the transformation of the times. With the networking of social operation mode, the diversification of social risks, the decentralization of social rights, the stratification of social identity and the high-end of social welfare, China should always adhere to the road of socialism with Chinese characteristics, give full play to the advantages of the socialist system, take root at the basic level, and explore the road of social security development that condenses Chinese wisdom. Based on the deep understanding of the transformation of main social contradiction, the theory of social security system should pay more attention to social cooperation on the basis of state responsibility, stress social empowerment on the basis of respecting rights, underline risk prevention and control on the basis of social assistance, and promote the transformation of the security object from being regarded as a burden to a resource to be developed. It is supposed to show concern for the basic function of social security and also emphasize economic and social development more. The optimization direction of the social security system under the transformation of main social contradiction should be demand-oriented, risk-based and aimed at common benefit. At the same time, it ought to be committed to increasing social services, highlighting the spiritual and cultural needs, and carrying out the precise design and implementation of the system.

Key words: main social contradiction, society with risks, social security theory, social security system