Aluminum nitride (AlN) has become an ideal heat dissipation material in the field of electronic packaging due to its high thermal conductivity and excellent thermal stability, but its strong covalent bonding properties make low-temperature sintering and densification difficult. The microstructure evolution of pure AlN under microwave irradiation was studied using synchrotron radiation computed tomography technology. The experiment found that the density distribution of AlN samples after microwave sintering was uneven: the density gradient decreased from bottom to top in the vertical direction, and was uniformly distributed in the horizontal direction. Proposed the "Coulomb force" driving mechanism and explained the regulatory effect of microwave electric field on density distribution - the positive feedback loop of heterogeneous charge attraction between vertically oriented AlN particles accelerates local densification; Horizontal particles are subject to repulsive forces from various charges, achieving uniform spacing. This mechanism provides a solution for microwave sintering to prepare AlN ceramics with uniform density distribution, which has reference significance for optimizing the process of high thermal conductivity electronic packaging materials.