Abstract: Three-dimensional finite element models of composite shot peening with large diameter (0.300 mm) and small diameter (0.178 mm) shots and of single shot peening were established and verified. The effect of composite shot peening on spurface integrity of 300M steel was studied, and was compared with that by the single shot peening. The results show that the simulated residual stress distribution in surface layer of 300M steel after peening was basically consistent with test results, and the relative errors of the maximum residual stress and the surface roughness were 3.57%, 2.56%, relatively, which verified the accuracy of the model. A residual compressive stress layer appeared in 300M steel surface layer after composite shot peening. With the increase of depth, the residual compressive stress increased first and then decreased. The higher the coverage ratio proportion of large-diameter shots, the greater the maximum residual compressive stress and the higher the surface roughness. The maximum residual compressive stress and the thickness of residual compressive stress layer after peening with composite shots were larger than those after peening with small-diameter single shots and were close to those after peening with large-diameter single shots. The surface roughness after composite shot peening was larger than that by small-diameter single shot peening, and smaller than that by large-diameter single shot peening.