Abstract: Caprock is one of the main reservoir-controlling factors of the Pingbei gentle slope belt (PGSB) in the Pinghu structural belt of the Xihu Sag. To find out the distribution law of high-quality caprocks and its role in reservoir control in the PGSB, 3D seismic data covering an area of 1200 km² and 23 exploratory wells in the PGSB were analyzed by well-constrained seismic inversion technology, and the development of the caprock in PGSB was analyzed. Results show that there is a set of regional mudstone caprock S1 on average porosity of 1.5% in the upper Pinghu section of the PGS, thus most of the oil and gas generated from the source rocks of the Pinghu Formation could be retained in the Pinghu Formation. The time window of the upper and lower 50 ms of the T₃¹ interface was selected to produce the time thickness map of the S1 caprock, which objectively showed the overall appearance and distribution characteristics of the caprock, and pointed out that the poor local sealability caused missing of the caprock in wells of L1, T2, W4, B2, etc.. Meanwhile, considering the influence of faults on the sealability of the caprock, it was pointed out that when mud smear factor was less than 1.8, it had good sealability for oil and gas. Overall consideration of caprock thickness, shale content, faults, and other factors, the sealability of the S1 caprock in the PSG belt could be classified and evaluated for providing guidance to oil and gas exploration in the area in future.