Abstract: Drop hammer method is the main method to determine the coefficient of coal hardiness. In the actual measurement process, the problem that existed in the method of determining the volume of coal particles in a certain range was pointed out. Based on the analysis of measurement process and principle, the influence of impact crushing times on particle size distribution characteristics of granular coal was studied by particle size distribution function and gradation curve. The results show that with the increase of impact crushing times, coal sample is crushed smaller and smaller, and the effect of impact crushing energy, characteristic particle size of granular coal and the uniformity are also decreasing gradually and tends to be constant. There is a critical impact number that makes the particle size distribution constant. The uniformity coefficient increases approximately in a line with the characteristic particle size. The non-uniformity coefficient and curvature coefficient of the gradation curve of granular coal show a logarithmic attenuation relationship with the characteristic particle size.