Base on speckle formation mechanism in laser projection display system, two key parameters that influence speckle suppression are theoretically analyzed, namely the number N of statistically independent speckle images and the number M of projection-lens resolution elements within a single imaging-lens resolution element on the screen. The increasing value of the parameter M or N can effectively reduce the contrast of the speckle image. However, although the number N of independent speckle images projected on the screen tends to infinity, the contrast of the speckle image can only be reduced to 1/ M . By using a rotating small diffuser to generate independent speckle image, the degrees of speckle suppression are experimentally compared without projectionlens and with projection-lens in simplified projection system. And the effects of the change of the F-number of the projection-lens or the F-number of the imaging-lens on the parameter M and the speckle suppression are experimentally analyzed. The results demonstrate that for the same value of N and under the same test conditions, the contrast of the speckle image increases from 0.146 to 0.427 and the speckle is more serious due to the presence of the projection-lens, and the increase of the F-numbers of the projection-lens can further deteriorate the speckle image, which will be a challenge for the miniaturization of laser projection system.