Abstract: Based on the qualitative analysis of the faults on the Pinghu slope of Xihu Sag, we conducted the semi-quantitative study of their sealing capacity in this paper. According to the lateral and vertical framework of the faults, the fault system in the Pinghu slope can be divided into four fault zones, i. e. the "K" synthetic fault zone, "W" antithetic fault zone, "P" synthetic fault zone and "N" graben-type fault zone from north to south. Results showed that the fault sealing capacity was higher in the middle part of the slope rather than in the south and north parts. The sealing capacity of most "K" synthetic fault zone and "W" antithetic fault zone was high. However, due to the long-term activity of the main Pinghu fault, the fault sealing capacity became ordinary as a whole. As the result of the weak compressive inversion of the Longjing movement by the end of Miocene, the sealing capacity of the "P" synthetic fault zone was fine. But the fault sealing capacity of the "N" graben-type fault zone was low, due to the long-term activity of the main Pinghu fault and the occurrence of the faults in subsidence period.