Sodium erosion has become a major factor affecting the durability of electrolytic cathode carbon block, therefore it is of significance to study the diffusion process of sodium in carbon block. In general, the carbon block can be considered as a multi-phase composite with carbon as aggregate and asphalt as binder, thus the sodium diffusion process might relate closely to the microscopic structure of the carbon block. First, seven microstructure models of cathode carbon block could be established by means of software Matlab in consideration of the different particle distribution and the amount of the carbon aggregate of various shapes such as circle, ellipse and polygon, and then which were through igs model file format introduced into ANSYS to establish a two-dimensional finite element numerical model. The ANSYS thermal analysis unit was used to simulate sodium diffusion process which based on the similarity of diffusion equation and the heat conduction equation, and analyzed the influence the size distribution, the amount and the shape of the carbon aggregate on the sodium diffusion process. The results show that the carbon aggregate shows stronger barrier effect to the sodium diffusion rather than the asphalt. For the carbon block with narrower range of the particle size distribution, lower roundness of the aggregate particles and higher amount of the aggregate, the sodium diffusion rate is slow down over time. The sodium diffusion rate is the slowest for the carbon block with 80% circular aggregate and the aggregate size distribution within a range 0.003~0.006 m. Furthermore, the above simulation results agree fairly well with experimental ones which proved the accuracy and reliability of the simulation.