Abstract: Turbopump is the core component of liquid rocket engine, operating in high speed, low temperature and harsh environment, its safety and reliability are determined by its sealing system. In the cryogenic environment, the dynamic viscosity of cryogenic medium such as liquid hydrogen and liquid oxygen is low, and it is easy to vaporize phase transition, which is easy to cause the mechanical seal face deformation, dry grinding and seal instability, which seriously affects the safety and reliability of turbopumps and even rockets. Aiming at the seal instability problem caused by the vaporization of cryogenic low-viscosity medium in liquid rocket engine turbopump, a phase change lubrication model of hybrid hydrodynamic-hydrostatic mechanical seal was established based on the homogeneous boiling model. LOX was used as the sealing medium to analyze the phase change characteristics of the fluid film between the seal faces, and the effect of operating conditions on the sealing performance was studied. The effect of working parameters (speed, pressure and temperature) on sealing performance were studied, and compared with the classical liquid lubrication model. The results showed that: The numerical solution method of the homogeneous boiling model based on the finite element method can be effectively used for the numerical analysis of the mechanical seal of the cryogenic oxygen medium with the hydrodynamic-hydrostatic mechanical seal, and has the simulation capability of the gasification phase change region of the liquid oxygen medium in a large range, and could be used for the calculation and evaluation of the mechanical seal performance of gaseous oxygen and liquid oxygen medium. When the oxygen in the sealing cavity was liquid, with the increase of temperature, the LOX phase change on the end face of the mechanical seal mainly occurred in the downstream side of the equalizing tank. The phase transformation could enhance the static pressure effect of the fluid film and can greatly reduce the mass leakage rate of dielectric oxygen. The spiral groove on the upstream side of the pressure equalizing tank has the upstream pumping function, which provided a good hydrodynamic pressure effect while pumping the liquid oxygen in the pressure equalizing tank to the upstream sealed chamber and plays the role of liquid film bearing. With the increase of temperature, the hydrodynamic pressure effect is slightly weakened. Compared with the classical liquid film lubrication model, the mass leakage rate of oxygen under the phase transition was 10% ~ 20% of that under the liquid film lubrication model. the variation law of the performance of the combined mechanical seal based on the homogeneous boiling model considering phase change is obviously different: Under the former model, the opening force and upstream pumping rate decreased linearly with the increase of temperature, while under the homogeneous boiling model, the opening force and upstream pumping rate showed a complex nonlinear change law, which was mainly due to the nonlinear vaporization characteristics of liquid oxygen. When the sealing cavity was filled with liquid oxygen, the phase change helps to improve the opening force of the fluid film. The opening force and temperature of the end fluid film showed a nonlinear change law. In the study range, when the spiral groove depth was 12 microns, the dynamic and static combined mechanical seal had better fluid film opening force and upstream pumping rate. Because of the isolation of the pressure equalizing groove, the spiral groove structure of the upstream side has no direct effect on the phase transformation and leakage characteristics of the downstream side. In the design of sealing face structure, the upstream spiral groove structure and the downstream sealing dam could be designed separately. The research results could provide reference for the design of cryogenic medium end seal of turbopump.