Numerical method of phase transition in vacuum plume is studied to research the contamination effects of infrared and erosion caused by ice crystal particles produced from vapor in plume. Based on two way coupled DSMC method used in rarefied two phase plume, the numerical method of phase transition in vacuum plume is modeled by using classical nucleation theory. The vapor spontaneous condensation in a Laval nozzle test cases are simulated, then the classical nucleation theory is modified by setting nucleation rate a correction factor of 103. The simulation results are good agreement with those of experiment. 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 obtained. It is show that there are a number of ice crystal particles because of phase transition in plume field, and there are a certain amount of crystal in back flow with the unlimited expansion in vacuum. There are some difference of number density and size distribution of particle from gas phase. It is need to study the contamination effects of plume.