Abstract: In order to improve the propulsion performance of existing underwater hydrofoil robots, the Taguchi experiments, neural networks and CFD are combined to systematically study the effects of aspect ratio, heaving amplitude, pitching amplitude and flapping frequency on the propulsion performance of a three-dimensional NACA 0012 hydrofoil. First, the parameter combinations for CFD simulation are determined by the Taguchi method. Next, CFD simulations are performed and the results are used as the training set of the neural network. Then, the neural network is trained and used to predict the CFD result. Finally, the mechanism for the optimal propulsion performance at the optimized parameter combinations is analyzed. The results show that the aspect ratio, heaving amplitude, pitching amplitude, and flapping frequency can significantly affect the propulsion performance of the hydrofoil, among which, the flapping frequency (aspect ratio) has the greatest (least) influence on the propulsion performance. The maximum propulsion efficiency of the hydrofoil can reach 55.43% after the optimization of the neural network. Further analysis of the flow field structure around the hydrofoil with different parameters reveals that, under the optimal parameters there forms a stable vortex on the hydrofoil surface, which can stay on the hydrofoil surface for a long time during the flapping process. This is the intrinsic reason for the better propulsion performance at the optimal parameters.