The thermal evolution of organic matter in Pingbei area of Xihu Sag in the East China Sea Basin has obvious segmented characteristics in vertical direction, and the thermal evolution rate of organic matter in deep overpressure formation is obviously higher than that in shallow normal pressure formation. At present, most of the existing studies analyze the segmented characteristics of organic matter thermal evolution from the single factor such as vitrinite reflectance, and lack of systematic and comprehensive research, and do not analyze the factors that may cause this phenomenon. On the basis of previous studies, this paper makes a comprehensive analysis of the organic matter composition characteristics, the influence of underground fluid, the change of thermal conductivity and the influence of overpressure on the thermal evolution of organic matter in Pingbei area of Xihu Sag. The results show that the depth of overpressure development is in good agreement with segmented depth of the thermal evolution of organic matter. Overpressure is the main factor causing the vertical segmented characteristics of organic matter thermal evolution in Pingbei area of Xihu Sag.