Abstract: Foam drilling has been widely used both at home and abroad, but the method for optimized design of hydraulic parameters has not been improved. This paper concluded the method for optimized design of hydraulic parameters for foam drilling based on multi-phase flow theory and sand carrying theory, and through theoretical calculation and analysis. That is, to determine the optimum gas/liquid ratio through the linear relation between critical gas/liquid ratio and wellhead backpressure; to determine the optimum wellhead backpressure through minimum bottom hole pressure in different depths; to determine the optimum foam flow rate by combining the optimum gas/liquid ratio, optimum wellhead backpressure, bottom pressure and cuttings concentration; and finally to determine the optimum gas injection flow rate and liquid injection flow rate according to optimum foam flow rate. Example calculations were conducted for optimum hydraulic parameters corresponding to 200 mm hole diameter, 114.3 mm out diameter of drill pipe, 1 980 m well depth and 9 m/h, 18 m/h and 27 m/h drilling rate. The optimized method presented in this paper has a practical guiding significance for optimizing hydraulic parameters of foam drilling.