The electric heating tube is constructed through compaction of electrical-grade magnesium oxide (MgO) powder particles. The stacking state of the particles exerts a significant influence on the thermal conductivity and insulation performance of the electric heating tube. Porosity is a critical index for characterizing the stacking state of particulates. Consequently, this paper presents a series of studies on the porosity of MgO powder particles. The discrete element method (DEM) is employed to establish the particle contact model and charge transfer model, in conjunction with numerical simulation. This approach is utilized to analyze the falling powder process of MgO powder particles in the electric heating tube. The study investigates the influence of the particle gradation on the porosity of MgO powder particles with particle sizes of 150 μm, 250 μm, and 350 μm. Additionally, the Kriging agent model is employed to predict the optimal particle gradation. The findings indicate that, in comparison with the single-size particle gradation, the multi-particle gradation can effectively reduce the porosity of the particles. The optimal particle gradation, in terms of mass fraction ratio, is determined to be 45.7: 32.3: 22, corresponding to a predicted minimum porosity of 35.91%. The discrepancy between the predicted and actual simulation outcomes is found to be 0.039%.