When coherent light scattered from a rough surface,speckle is detected by an intensity detector. Speckle severely affects the image quality and therefore must be suppressed. Speckle suppression can be achieved by averaging a number of speckle configurations produced by vibrating a diffuser. The method is applied in a liquid crystal on silicon (LCoS) based laser projection system,and experiments are performed to study the relation between speckle contrast and diffuser movement. After suppression,speckle contrast on detector is reduced from 0.49 to 0.065. Experimental results indicate that larger amplitude and frequency of diffuser vibration,as well as smaller diffuse phase unit size,can reduce speckle contrast on the detector.