In the exhausting process of screw vacuum pump,there is internal air leakages from the high pressure discharge side to the low pressure suction side,which affect the pumping speed and the ultimate vacuum seriously.Therefore,it is signifcant to research the internal leakages for predicting performance of the pump and selecting the parameters of rotors.The clearance between rotors and case constitutes internal leakage paths of the pump.The single thread and fxed pitch screw vacuum pump is studied and the formulas of area utilization ratio,the theoretical pumping speed and the ultimate vacuum are deduced according to the equations of tooth profle.To calculate the leakage,the fow in the clearance is analyzed,and a simplifed model of poiseuille and couette fow in the circumferential and radial gap and holes fow in the fank gap is created.The program is wrote to analyzed the leakage,and the infuence of different outer screw radius R,inner screw radius r,base circle radius r0 and different screw pitch T to the leakage,pumping speed and the ultimate vacuum is studied.Results show that: Increasing r0 can effectively reduce the total leakage,improve the ultimate vacuum,but will reduce pumping speed slightly; With the increase of T,the total leakage can be reduced,pumping speed and ultimate vacuum can be increased; The pumping speed and the total leakage will be increased with the increasing of R,while the ultimate vacuum increases at frst and decreases in the end; Increasing r will reduce the total leakage and the pumping speed,the ultimate vacuum will be reduced too; Under the condition of specifc pumping speed,the minimum leakage and the highest vacuum can be achieved with the ratio of r and R equal to 0.65.