A restricted diffusion model was set up to investigate the simulation and optimal design of the residual oil hydrodemetallation in a cocurrent moving-bed reactor. The system parameters examined included the Thiele modulus, oil retention time, flow rate ratio and pore size. Three types of catalyst with different activity profiles were investigated, i.e. uniform, linear and two-step. Several figures were presented to predict the outlet metal concentrations and deposit conditions of metal sulfide in the outlet catalysts. Based on minimizing the outlet metal concentration, the optimal catalyst pore sizes and their corresponding optimal reactor lengths were determined. The results show that the optimal pore size increases with increasing values of the Thiele modulus. The usage of nonuniform activity catalysts and multistage reactor can improve the reactor performance. With the values of oil retention time and flow rate ratio properly chosen, it is found that the one-stage reactor with catalysts under the optimal conditions can significantly improve the reactor performance.