Japan’s social security system started in the 1920s，but it was modernized after World War Ⅱ. The modernization of Japan’s social security system has gone through five different stages：the establishment period，the completion period，the transformation period，the reform period and the deepening period，and has gone through a process from “saving poverty” to “preventing poverty”，from “expanding” to “adjusting”，and then reforming and perfecting. Judging from the path choice of social security system modernization，social insurance and social assistance are mainly adopted to realize the systematization and full coverage of social security system. In the process of modernization of social security system，we have encountered some reform dilemmas，such as the challenges brought by population decline and the aging of fewer children，the shortage of financial resources for social security，and the sustainability of social security system. However，through a series of social security system reforms，we have achieved certain results. The social security system has played the role of “stabilizer” and “safety net”；Realizing the redistribution of social resources，narrowed the gap between the rich and the poor，and promoted the stability and development of social economy.