Abstract: The water-saturated coal samples were heated by microwave for 0 s, 50 s, 125 s and 200 s, respectively. After that, a test system consisting of SHPB and high-speed photography apparatus was adopted to study the variation of mechanical parameters, the development of cracks during dynamic loading and the characteristics of breaking lumpiness. The research results showed that, microwave heating had time effect on the damage of water-saturated coal rock. After heating for 125 s, the peak stress and peak strain of coal sample decreased greatly, and the lumpiness after breaking was more uniform and finely. In addition, the longer the microwave heating period, the earlier the coal rock will break up. The reason for the above phenomenon is that the moisture in the coal rock vaporizes rapidly stimulated by microwaves. With the process of heating, the steam pressure inside the coal sample rises continuously and reaches its maximum after heating for 125 s, resulting in the development and expansion of pores and micro-cracks inside the coal rock, thus leading to the deterioration of the mechanical properties of the coal sample.