The traditional smoke theory considers that the smoke particles will sedimentate due to gravity after forming of smoke screen, and then the particles reach a final sedimentation velocity thanks to the effect of air-resistance and buoyancy, and the equation can be solved by the Stokes pellet resistance theory and often used to analyze the performance of smoke. In this paper, through outfield experimental research and theory analysis, the Stokes final sedimentation velocity formula is not demonstrated to adapt to analyzing performance of smoke.