The accelerating characteristics of copper particles in the process of vacuum cold spraying were investigated based on numerical simulation of gas-particle two phase flow with Fluent, and the influences of ambient pressure, stand-off distance ( SoD) , inlet temperature and diameter of particles on particle impact velocities were analyzed. The results indicate that the ambient pressure is the key factor affecting the particle impact velocities, and the velocity curve for the small particles with diameters smaller than 1 μm is parabolic with the change of the ambient pressure, while no significant change for the big particles; The higher impact velocities may be obtained with suitable SoD; The impact velocities increase with the increment of inlet temperatures for the small particles, but which does not happen for the big particles with the same conditions; The minimum_particle size can be reduced from micron size to submicron size in the process of vacuum cold spraying, but too smaller particles cannot obtain enough high impact velocities so as to be deposited.