Based on the pyrotechnic and powder smoke cloud, the effect of vacuum pressure on the expanding velocity during the smoke cloud forming was studied experimentally with the method of the expansive velocity obtained by testing the increases of the cloud edge size. Results show that the expanding velocity increases with the increases of vacuum from 0.10 MPa to 0.04 MPa, and the velocity increases about 0.03 times when the pressure decreases 0.02 MPa.