Objective The flow behavior of gas and the particle impact velocity are directly affected by the vacuum degree in the process of vacuum cold spray. The influences of vacuum degree on gas and particle flow properties were studied. Methods The structure of the vacuum cold spray system was determined, and the gas-solid flow in vacuum spray was studied based on FLUENT. The influences of vacuum degree on the flow field and particle impact velocity were explored through numerical simulation, and the gas-solid flow properties under the same pressure ratio were studied. Results The numerical research indicated that when the inlet pressure was constant, the axis velocity, the density and the temperature of gas in the nozzle were independent of the ambient pressure, whereas in the jet flow region, lower ambient pressure led to lower fluctuation of the gas velocities and lower gas density. However, when the gas reached the substrate, its temperature was close to the inlet temperature in all cases. The ambient pressure had obvious effect on the impact velocity of the particle with large diameter, and the impact velocity first increased and then decreased with increasing ambient pressure, and the optimal ambient pressure could be determined by the gas contour and its density. When the pressure ratios of the inlet and outlet of the nozzle were the same, the gas contours in the nozzle and the jet flow region were almost the same, and the curves of the gas axis velocities almost coincided. While the particle velocities before the substrate were different, the lower the ambient pressure, the higher the particle velocity, which was more beneficial to coating formation. Conclusion The influence of vacuum degree on the gas-solid flow in vacuum spray was clarified based on the CFD, which laid the ground work for the preparation of coating.