Abstract: Based on the numerical solution of flow-solid coupling mathematical model, the mechanical mechanism of strata cracking after coal unloading is explained from the perspective of seepage breaking coal. Through the collation and analysis of the calculated data, the mechanical quantitative conditions for the initial failure of the coal body under the action of seepage were obtained, and the mechanical conditions for the continuous advance and termination of the fracture failure were qualitatively described. The results of numerical calculation show that the tensile strength, initial adsorption pressure and unloading rate of the coal body determine whether the initial failure can occur and the thickness of the fractured body. Whether the spalling failure can continue to advance is mainly determined by the pressure relief condition of undamaged coal body and the pressure change trend in the spalling space. The former can be obtained quantitatively through numerical calculation, while the latter is related to the thickness and crushing degree of the crack body after the spalling failure.