Abstract: During the rapidly expansion of vacuum gas plume, the condensation phase transition is easy to occur for species such as H₂O and CO₂ in gas phase. Numerical simulation method of phase transition process in vacuum plume is studied to research the contamination effects of infrared and erosion caused by ice crystal particles produced from vapor by phase transiting in plume. Based on two way coupled direct simulate Monte Carlo (DSMC) method which simulates the momentum and energy exchange between phases used in rarefied two phase plume, the numerical method of phase transition in vacuum plume is modeled by using classical nucleation theory and droplet growth theory. The experimental cases of vapor spontaneous condensation in a Laval nozzle are simulated. By comparing the results between numerical simulation and experiment, the classical nucleation theory is modified by setting a correction factor of 10³ for the nucleation rate, then the simulation results which are good agreement with those of experiment can be obtained. This means that the modified classical nucleation theory can be used in numerical simulation of rarefied flow. The vacuum plume with phase transition of a thruster is simulated, and similar result with CFD in kernel flow is acquired. It is show that there are a number of ice crystal particles because of phase transition in plume field. In plume kernel flow, the maximum of ice crystal number density reaches the order of 10¹⁵ m⁻³, and the diameter is on the order of about 10⁻⁸ m in the simulated case. There are a certain amount of ice crystal in back flow because of the plume unlimited expansion in vacuum, with the number density on the order of 10⁷ to 10¹⁰ m⁻³, and the diameter on the order of 5.0 × 10⁻¹⁰ m. There are some distinctly difference between particle and gas phase about number density and size spatial distribution, and it is needed to study the related contamination effects of plume.