Abstract: In order to improve the rock-breaking efficiency of raising boring cutters under hard brittle rock, it is necessary to obtain the optimal drilling pressure. The analysis of the size of rock ballast is an important means. The cone-tipped cutter is used to break hard brittle marble samples on the LCM testing machine, and the rock ballast is analyzed by means of the Rosin-Rammler distribution. By comparing the cumulative probability curve of size distribution of rock ballast, the characteristic size and specific energy under multiple drilling pressures, the relationship among drilling pressure, the characteristic size of rock ballast and specific energy are obtained. The optimum drilling pressure of marble samples is 34.45 and 35.19 kN respectively when the characteristic size is the largest and the specific energy is the smallest, so it is suggested that the drilling pressure should be set as 35 kN. The size distribution between hard brittle marble and granite is also analyzed under the same drilling pressure, and the characteristic sizes are obtained under different uniaxial compressive strengths. The results show that the characteristic size can reflect the rock-breaking effect of cone-tipped cutter, and the rock-breaking process conforms to the new surface theory of Rittinger. The characteristic size and specific energy are inversely proportional. Within a certain range of drilling pressure, the characteristic size of granite is larger than that of hard brittle marble, and the difference of characteristic size increases with the increase of drilling pressure.