Abstract: The displace pressure difference between caprock and reservior, abnormal pore fluid presure, abnor mal gas-bearing c0ncentration, sealing ability of faults, thickness of mudstone, and depositional environment were choosed as parameters in evaluating the microcosmic sealing ability and macroscopic distrlbution of mudstone caprocks. By grading and weighting of the parameters, in accordance with their development and efficiency, the weight for comprehensive evaluation of sealing ability of mudstone caprock was calculated by weighted average method. It was applied to evaluate the sealing ability of mudstone caprock in member N2 of Daqing placanticline and the area to its west. The result was consistent to the actual geological conditions in the region. It indicated that the method was practical for evaluation of mudstone caprock.