Laser speckles create a bottleneck that hinders the development of laser projection display technology. Based on the compound speckle contrast analysis method, the parameters affecting the composite speckle lining ratio in a laser projection system are analyzed. These parameters include the number N of independent speckle samples and the number M of scattered light wave coherent elements within a single imaging-lens resolution element on the projection screen. The occurrence of speckles in the laser projection system can be reduced by changing parameters N and M. These changes can be achieved by adjusting the numerical apertures of the projection-lens and imaging-lens, projection distance, and observation distance. In the simplified projection model, the effects of the random-diffuser spot area and projection-lens numerical aperture on the speckle contrast in laser projection imaging were experimentally analyzed. The results reveal that for a certain size of the projection lens, the contrast of the secondary speckle pattern on the scattering screen can be reduced by increasing the area of the diffuser spot. The occurrence of speckles increases when the numerical aperture is reduced, thereby posing a challenge for the miniaturization of the laser projection system. The findings of this work can provide guidance for the use of a rotating random diffuser in the design of speckle suppression systems for laser projection imaging.