Social Capital is used by Putnam as a key conception to explain the social change in Italy and America. But the Operating Mechanism of Social Capital is not presented clearly. This article tries to explain this mechanism in a new way by using Habermas’ theory of communicative action. Two levels (Communicative Rationality and Critical Theory) and three aspects (The Lifeworld and The Two Major Systems, Political and Economic) are provided here to reach the root substance of social capital and its Operating Mechanism. And further study on Chinese society today should be carried on in the future.