Abstract: Foam drainage technology is often adopted in the late stage of shale gas development. After its produced foam drainage wastewater is mixed with fracturing flow-back fluid, the introduced surfactant increases the water treatment difficulty greatly. In this situation, the field demand cannot be satisfied if only water treatment technology is optimized. In order to decrease the fracturing flow-back fluid treating difficulty, it is necessary to search for a solution again from the very beginning. The selection of surfactant for foam drainage was studied from the perspective of the treating chemical PAC. The fracturing flow-back fluid in the southern Sichuan Basin was taken as the research objet to investigate the influences of four kinds of surfactants on the treating chemical PAC while measuring Zeta potential of suspended materials to understand the influence mechanisms. Then, the foamability was evaluated. Finally, the surfactant with less influence on flocculation and good foamability was selected out and applied on site. It is indicated that the influence of surfactant on PAC is ranked as anionic surfactant>nonionic surfactant>zwitterionic surfactant≈cationic surfactant, and dodecyle ethoxy sulfobetaine (DESB) is the optimal surfactant. After the foam drainage technology is applied on site, the average daily concentration of downstream fracturing flow-back fluid treating chemical PAC is reduced from 400-600 mg/L to 300-400 mg/L while the foam drainage demand is satisfied. Obviously, the application effect is good and worth popularizing.