On the occasion of the 40th anniversary of China's Reform and Opening-up，this paper reviews the difficult process and great achievements since the establishment of social security system，and analyzes the origin and motivation of reforming the social security system.The paper argues that the main contribution of social security to economic and social development is by which enterprises are made become real market players and individuals are made “free laborers”.Furthermore，Social security has become the regulator in market economic operation and the safety net of social stability.Moreover，this paper summarizes the historical experience of China's success in social security over the 40 years，as well as the major problems left over，which are argued as a paradox that all problems are stemmed from “extensive trial and error”.At last，it put forward 4 viewpoints for the orientation of social security reforms in the future，namely that China's social security needs to be transformed from high speed expansion to high quality development，that the uneven and deficient problems obviously existing in social security development should be resolved as soon as possible，that the top-level design of social security carry a big weight in the supply-side structural reform，and that the social security reform's adhering to “ the principle of actuarial balance” is of far-reaching significance.