Based on the theory of the speckle and the self-mixing interference in laser diode (LD), Fabry-Perot (F-P) cavity model is given. The dynamics solution of a laser diode is obtained in the case of a rough surface after studying and analyzing the self-mixing speckle interference (SMSI). The Gaussian correlation random surface and its speckle field on the Fraunhofer plane as well as superposition of speckle filds and interference field are simulated, and theiry statistical characteristics of the optical intensity are also analyzed. Compared with the known experiment, the results of simulation agree with the known statistical properties. The output gain variation of a LD and its probability density function are obtained when the rough surface is moving across the light beam. The effects of illumination width and external cavity length on the changes of output gain of a LD are discussed. The experimental results agree with the simulation calculation.