The sealing ability of faults is one of the key factors determining whether water flooded and depleted gas reservoirs can be successfully transformed into underground gas storage. Taking the Y21 water flooded depleted gas reservoir in Dong-ying depression as the research object, this study conducts fault sealing evaluation from both static and dynamic aspects. By using geological research methods such as seismic inversion and attributes analysis, rock sample analysis and testing, the lower limits of sealing ability for fault opening pressure, fault mudstone smear coefficient, breakthrough pressure of fault zone mudstone, and fracture density of fault zone were comprehensively determined. A comprehensive evaluation method and indicators for static sealing of faults were established. Due to the periodic impact of changes in injection and production pressure on the geostress field of underground gas storage, a numerical simulation method of injection and production gas circulation geostress field was used to evaluate the dynamic sealing of faults. And that identified the weak zones of dynamic sealing of faults. Based on the research results of comprehensive static and dynamic sealing evaluation, it is believed that the sealing performance of the two faults on the south and north boundaries of Y21 gas storage is good, which meets the needs of gas storage construction. This study has certain guiding significance for the reconstruction of underground gas storage from water flooded depleted gas reservoirs and aquifers reservoirs.