The non-contact mechanical seal of high-speed turbopump in the liquid rocket engine operating under harsh conditions such as low-temperature， high-speed， and high-pressure， is easy to produce gas-liquid two-phase flow. The mechanism of inducing two-phase flow and the effect of phase transition on seal performance are still unclear， so it is urgent to develop relevant theoretical models and perform experimental test. Based on Laminar flow and Mixture multiphase flow models， this research， taking a non-contact hydrodynamic spiral groove mechanical seal of high-speed turbopump as the object， studies the cavitation phase transition characteristics of fluid in the seal gap under low temperature high-speed harsh conditions. The main seal performance parameters， such as leakage， opening force and proportion of internal gas phase volume， are obtained， and the influence of the working parameters on the two-phase flow characteristics and the phase transition process with the low temperature liquid nitrogen as the sealed fluid is analyzed. The results show that with the increase of the film thickness， the volume of the fluid in the sealing gap increases， and the dynamic pressure effect decreases，which leads to a small change in the internal pressure， and therefore， reduces the cavitation phase transition possibility. With the increase of the film temperature， the proportion of internal gas phase volume increases， and decreases with the increase of inlet pressure. A series of low-temperature experimental tests are carried out， and the correctness of the theoretical models is verified. The study will help to understand the two-phase flow mechanism of low-temperature high-speed non-contact mechanical seal， which has important engineering value for improving the operational reliability and stability of reusable high-speed turbopump.