Abstract: In order to increase the built-in volume ratio of the screw rotors, a new design method of variable cross-section screw rotors was proposed in this paper, and variable cross-section conical screw rotors were constructed. The influence of geometric parameters on the performance of the proposed screw rotor was studied and compared with the traditional screw rotor with a constant cross section.Resultsshow that the tooth thickness distribution of the proposed screw rotor is more reasonable, and the direction of pressure increase is consistent with the direction of the tooth thickness increase. Compared with the suction volume and discharge volume of the traditional screw rotor, the suction volume of the proposed screw rotor is larger, and the discharge volume of the proposed screw rotor is smaller than that of a traditional screw rotor. Hence, the built-in volume ratio is improved. It also has advantages of ultimate high vacuum, good inter-stage sealing and high pumping rate.