Abstract: The sealing mechanism of the Huagang Formation caprock and its influencing factors in the central inversed structural belt of the Xihu Sag remain unclear up to date. In this paper, we used various methods, such as core observation, microscopic examination, X-ray diffraction, and the methods specially designed for study of lithology and diagenesis to assess the quality of a caprock and its influencing factors in order to know more about the caprock in the region. Breakthrough pressure testing was also made. The analysis of influencing factors suggests that the total thickness of the cover of flood plain facies in the Huagang Formation of the region belt is the largest, followed by the facies of the distributary bay, while the total thickness of the cover between waterways is the smallest. The study of diagenesis suggests that the diagenesis of compaction, cementation and metasomatism is constructive, while that of dissolution and shale contraction is destructive. Diagenesis has strong effects on the development of pores at different depths, which directly effects the sealing ability of the caprock. In the study area, the facies of flood plain is the most favorable facies, and cementation and compaction are the most favorable diagenesis for caprock development. Combined with the distribution characteristics of caprock and breakthrough pressure, a four-level system for caprock assessment is established in this paper.