Water block damage is likely to occur in tight gas reservoirs of Dabei-Keshen area and decreases production of gas wells. In order to investigate the mechanisms of water block and the corresponding measures, the water block index(WBI)is developed to appraise the damage degree of water block in approach of relative gas permeability, and composite surfactant system is optimized to clean up effects of water block damage through interfacial tension tests, wettability tests and spontaneous imbibition experiments. The results and conclusions are as follows: the average WBI for core samples from the targeted zone is 69.8%, belonging to the type of strong damage of water block. Sensitivity analysis shows matrix permeability and displacement pressure are in negative correlation with WBI, while water saturation, content of clays, fluid viscosity and interfacial tension are in positive correlation with WBI. In this work, thermo-stable surfactant systems JY-2(0.05% FS-31+15% methanol)and JY-3(0.5% HSC-25+15% methanol)are developed. JY-3 performs better in reducing WBI from 66.2% to less than 28.4%. Surfactants in the composite system contributes to reducing the interfacial tension and altering wettability, and methanol is beneficial to reducing water saturation through accelerating evaporation shortly. The synergy effect promotes the cleanup process of water block damage. The found mechanisms of water block damage and numerous experimental data provide us valuable insight on the economic and efficient development of gas fields.