Clearance leakage is the main cause of pump leakage. Aiming at the design and control of the key clearance, which determines the performance of screw pump such as clearance of barrel wall and meshing area, the model of differential pressure flow and shear flow of clearance leakage in pump cavity is established. The mathematical expressions of leakage from different clearance are obtained by analyzing the clearance leakage of barrel wall and meshing area. The influence of the rotation speed and pressure difference between inlet and outlet on flow characteristics and volumetric efficiency characteristics of screw pump is revealed by theoretical calculation and experimental analysis of twin screw pump. Based on these results, the rotor design theory of the screw pump as well as the performance of the screw pump can be improved.