In order to study the ripple phenomena and its causing mechanism on material surface that caused by the laser irradiation, the pulse generated by neodymium glass laser was used to shock the specimen of magnesium alloy AZ91 covered with black paint as the absorption layer. The threedimensional surface measurement instrument is applied to measure the shocked region of the specimen surface after shocking. And the ripples phenomenon is observed in the shocked region from the measurement results. The surface morphology, as well as the distribution of the surface ripples, is observed and described; and the relation between the characteristics of the ripples and the laser energy is analyzed. It can be concluded that the characteristics of the ripples are affected by laser energy. Based on the effects of the plasma to the specimen and the grating effects caused by the mechanism of coherent stimulated light scattering within plasma, the process of generating thermal microdisturbance on the sample surface by coupling the thermal conductivity, the thermal radiation and the laser irradiation is discussed, and then the formation mechanism of the surface ripples caused by the nonequilibrium state of the surface thermal microdisturbance is discussed.