Abstract: The heterogeneous spatial distribution of bulk material stockyards is one of the key factors constraining the accurate prediction of spontaneous combustion behavior in storage systems such as grain silos and coal piles. In order to overcome the limitations of the homogeneous parameter assumption in existing research on self-heating of bulk materials and to improve the accuracy of spontaneous combustion prediction in bulk storage systems, this paper focuses on the spatial distribution of porosity within the bulk material. By considering the formation process of “falling-collision-accumulation” in bulk materials, a numerical reconstruction approach using the discrete element method to simulate the formation of bulk materials is proposed. Based on this, the paper studies the distribution law of non-uniform porosity within the bulk material under different particle sizes and dumping heights, discusses the influence of equivalent particle size and falling height on the distribution law of non-uniform porosity within the bulk material, quantitatively characterizes the distribution characteristics of porosity within the bulk material, and compares the impact of the homogeneous assumption on the spontaneous combustion law of coal piles. The results show that: ① The overall porosity of the bulk material slightly increases with the increase of dumping height, but the influence of dumping height on the distribution law of porosity within the bulk material is not significant. The average change in porosity when the dumping height increases from 0.55 m to 0.95 m is only 5.1%. ② Influenced by the gravity and compression of particles above, the porosity within the bulk material gradually decreases from high to low along the vertical direction, but the porosity of large particles near the ground will increase, reaching the lowest value at a height of about 3 cm above the ground. ③ The porosity within the bulk material is approximately uniformly distributed along the horizontal direction, with an average value of about 0.45. ④ The homogeneous assumption has no significant effect on the natural ignition period, but after adopting the homogeneous bulk material assumption, the center of the self-heating zone migrates 0.3 m towards the surface of the bulk material, and the length of the self-heating zone is shortened from 10.2 m to 3.1 m, significantly underestimating the risk and concealment of coal self-heating. This study provides a new research idea and direction for the analysis of typical heterogeneous structures in bulk storage systems, and is expected to break through the prediction deviation of coal self-heating caused by the difficulty in characterizing the heterogeneity inside large coal piles at the current stage, greatly improving the prediction accuracy of spontaneous combustion processes in bulk porous systems and making coal self-heating prevention measures more targeted.