Abstract: Understanding the internal working process of screw vacuum pumps is the foundation for designing high-performance screw rotors. In this paper, a modeling analysis of the working process of variable pitch screw vacuum pumps is carried out. Firstly, based on the geometric structure model of the simplified leakage channel, a time-based inter-stage leakage model of the internal leakage channel for the screw vacuum pump is constructed under two flow modes, namely Poiseuille flow and Couette flow, and three flow states, namely viscous flow, transitional flow, and molecular flow. Then, based on the assumptions of gas transportation conditions, a gas transportation model for the internal suction stage, compression stage, recoil stage, and exhaust stage is constructed. Finally, based on the inter-stage leakage model of the internal leakage channel and the gas transportation model, the internal mass, pressure, and temperature distributions, as well as the distribution law of leakage flow velocity, are revealed. The analysis results show that: (1) as the inlet pressure decreases, the internal mass decreases, the pressure decreases, the temperature increases, and the proportion of leakage mass in the total mass increases; (2) as the clearance decreases, the internal mass decreases, the exhaust gas temperature decreases, and the proportion of leakage mass in the total mass decreases; (3) during the suction and compression stages, the main leakage channel is the tip meshing clearance (concave clearance), and during the exhaust stage, the main leakage channel is the tip clearance. The research results have guiding significance for the design of screw rotors and the performance optimization of screw vacuum pumps.