With the expanding of three screw pump application, many occasions require the use of high pressure, at least up to 25 MPa. Because of the pressure increasing, the problem of leakage cannot be ignored. The meshing principle of the three screw pump was elaborated. According to differences of meshing form, the total leakage was divided into three leakage parts. For each part, an independent mathematical model was established and the leakage was decided by different clearance, gesture and revolution speed.The final results show that most amount of leakage exists in the spiral region of active and driven screws, and there are relatively small leakage between the bush wall and cylindrical surface. To meet practical requirements of volume efficiency, there are demands for machining accuracy of screws and bush in theory.