Research on effective source rocks directly affects the accuracy of identifying hydrocarbon resources, and indirectly affects the exploration decisions in petroliferous basins. Although the previous evaluation methods of effective source rocks vary relatively widely, a complete quantitative evaluation approach has not yet been developed. For that reason, we redefined the concept of effective source rocks based on the existing research results. Surrounding this definition, and guided by the hydrocarbon expulsion theory, the quantitative model called "two stages and three steps" method is established to predict effective source rocks. Its application in the Bozhong Depression indicates that among the four sets source rocks in the Bozhong Depression, the Member 3 of the Shahejie Formation (Es3) has the largest effective source rock thickness, and the Member I-Member 2 of the Shahejie Formation (Es1+2) is the second largest .The effective part of dark mudstone is only 30%-80% of the total volume and with the increase of buried depth and improvement of quality, the effective part increases. Comprehensive analysis indicates that the "two stages and three steps" method is a practical technique for effective source rock prediction.