Due to the inadequacy of the existing experimental techniques, it is difficult to observe the microstructure evolution during the sintering process in real time, resulting in a lack of in-depth understanding of the sintering mechanism of two-phase composite fuels. Therefore, it is greatly important to carry out theoretical simulation studies on the sintering process of composite fuels. In this work, a phase-field model of the two-phase sintering process of ceramic composite fuel is established, and the sintering process of UN-U ₃Si ₂composite fuel is simulated by using this method. The simulation results show that the surface deformation of the grains with higher surface energy is obvious during the formation of sintering neck. The final equilibrium dihedral angle formed by the two-phase double grains depends on the ratios of the grain boundary energy to the surface energy of the two phases. The phenomenon of large grains swallowing small grains do not occur between the two unequal double grains. Subsequently, the pore shrinkage and the properties of the trident grain boundary between the two-phase three-grain are investigated during the sintering process. It is found that the angle of the trident grain boundary formed by the two-phase three-grain deviates from 120°. The high-energy barrier at the grain boundary impedes the diffusion of the pore vacancies along the grain boundary, resulting in a slowdown of the pore shrinkage rate at the trident grain boundary. In addition, the simulation results of the microstructure evolution of two-phase polycrystalline sintered tissue with different volume fraction ratios show that the grain boundary diffusion plays a major role in the two-phase sintering process. The grain growth of the phase with a larger volume fraction is dominant, and the role of hindering the grain boundary migration between the two-phase grains exists. The phenomenon of grain migration exists among grains of the same phase.