Abstract: Based on the reaction of synthetic ammonia, a three-dimensional multicomponent reaction-diffusion model of catalyst particles was established and verified by COMSOL software. The results of model verification showed that there was little difference between the multicomponent diffusion model and the one-component diffusion model. The inner surface utilization of ammonia catalysts with different shapes was close to each other. In industrial reactors, the calculation of internal diffusion efficiency factor of ammonia synthesis catalyst could be carried out according to the uniform surface area of spheres. Based on the different positions of the reactor, the simulation results of the synthesis ammonia A301 catalyst show that temperature, particle size and reaction process are important factors affecting the diffusion efficiency of catalyst. The diffusion effects of different reaction stages are quite different. At the initial stage of reaction, when the reacting rate and the internal diffusion retardation is large, the internal diffusion efficiency factor can be improved obviously by reducing the particle size of catalyst. The results show that the efficiency factor is almost linearly negatively correlated to the particle size of catalyst. In the middle and late stages, when the reaction is close to equilibrium, the utilization rate of the inner surface of the catalyst can remain above 0.95, and the internal diffusion is not sensitive to the changes of temperature and particle size. In this case, the catalyst with an appropriate large particle size can be selected in the reactor in order to reduce the pressure drop of the catalyst bed.