The process of coal and gas outbursts (called outbursts for short) are commonly accompanied by coal mass spalling and fragmentation, but the generation and evolution of coal mass spalling in this process is unclear. In order to explore the dynamic evolution mechanism of coal mass spalling, this research constructs multi-layered combined coal-rock mass model. Based on the stress wave propagation mechanism, it’s found that outbursts initially start from the “weak layer”, which is located at the damage-destruction interface. Besides, there are two significant results by analyzing the process of unloading waves formed by sudden pressure relief of the excavation face. Firstly, as for the axial direction, due to the internal impact of the coal mass during the unloading wave propagation, the load shock wave is formed. And the tensile stress is formed on the free surface of the coal mass, which resulting in rapid damages to the coal mass and even the multi-layered spalling. Secondly, along the radial plane, permanent standing surfaces will be formed when the unloading wave catches up with the plastic load wave. Under effect of the unloading wave, internal impact will occur a series of load shock wave, resulting in the formation of multi-layered spalling in the radial plane. In addition, the attenuation law of the load shock wave generated by the internal impact in multi-layered coal mass is also analyzed, results showed that the thickness of coal mass spalling formed far away from the free surface will increase, and the calculation model of coal mass spalling thickness is constructed. Lastly, the three-dimensional dynamic damage path of multi-layered coal mass and the formation mechanism of spindle shape damage area in coal mass under external dynamic load disturbance are analyzed, which lays a theoretical foundation for elucidating the mechanism of outbursts.