In order to investigate the characteristics of secondary speckle trans mission in laser projection imaging,a test system is built.Semiconductor laser is used as light source to measure the speckle contrast ratio of secondary scat tering field produced by scattering screen on different roughness surfaces.The experimental results show that with the increase of the distance between the sca ttering screen and the observation plane,the speckle contrast ratio decreases f irst,then increases,and finally tends to be stable and constant.On the basis of the formation and propagation mechanism of speckle,the transmission characte ristics of laser speckle produced by this specific time coherent length light so urce in space are analyzed theoretically.The analysis shows that the minimum po sition of speckle pattern contrast is related to the scattering surface roughnes s,which can provide a reference for the measurement of surface roughness.