Abstract: In order to clarify the reasons of mechanics parameters reduction in the numerical simulation process, the strength size effect was taken as the starting point of the problem. The external appearance of strength size effect about strong bump-prone coal was studied by laboratory test. The theoretical formula of strength size effect was deduced based on the Weibull distribution of the weakest chain theory, and the inherent nature of strength size effect was revealed. The influencing factors of strength size effect were discussed by means of numerical simulation. The results show that: the bump-prone coal has obvious size effect. As the size (length, width and height) increases, the strength decreases, which was well in accordance with the exponential function relationship; The probability and statistical distribution can express the intrinsic nature of the intensity size effect, indicating the homogeneity m is the main influencing factor of the strength effect of coal-rock mass.Numerical simulation results show that when the size is constant, the strength of coal rock shows an upward convex curve with the increase of homogeneity, which can be divided into three stages: namely the rising phase of volatility, the stage of rapid growth, and the stage of gradual growth. With the increase of homogeneity, the strength tends to converge, and the limit value is the meso size strength of coal and rock mass;when the homogeneity increases, the effect of size change on the strength decreases.