Abstract: Heap leaching is a widely used solution mining technology that enables various kinds of low-grade ores to be processed economically. The solution flow characteristics are very important factors in the leaching process, and they influence both the overall recovery and kinetics of the system. The properties of fluid flow in porous media are associated with the pore structure, which is influenced by the grain size and shape. To study the influence of ore particle size gradation on the pore structure of the heap leaching system, a micro-CT scanning test was conducted in ore columns with two grain size gradation types, and images of the internal structure of the leaching columns were obtained. A 3D digital pore model of the two leaching columns was then established, and the spatial distribution characteristics of 2D and 3D porosity were analyzed. The pore network models of the two columns were then extracted from the reconstructed 3D binary pore structures using the maximal ball fitting method, and the effect of the ore particle size distribution on the pore throat radius, throat length, pore throat volume, shape factor, and coordination number was analyzed. The results show that the porosity of the column comprising well-graded ore particles is lower than the column with uniformly graded grains. In addition, the 2D and 3D porosities of the well-graded ores show a relatively high degree of heterogeneity compared to those of the more uniformly graded ores. The ore particle size gradation has a significant influence on pore size and pore connectivity, but it has a minimal influence on the pore throat shape factor. The number of large pores increases with a decrease in the amount of fine ore, and the pore throat radius, throat length, and pore throat volume also correspondingly increase. When the uniformity of ore particle gradation is enhanced, the proportion of isolated pores decreases and the proportion of the number of high coordination pores increases.