Flapping wing propulsion is characterised by high efficiency and mobility, and many scholars have conducted in-depth studies on birds, manta rays, and other creatures with fluttering wing movements in anticipation of producing robots with high bionic effects. For wing-fluttering motion, the amplitude and frequency of fluttering are the key parameters affecting its effect. In this paper, a manta ray imitation robot is fabricated with manta rays as the research object, and the resonance of the ray bionic robot at the optimal flap frequency is realised by adjusting its intrinsic frequency with the flap frequency under the optimal working condition, so as to achieve the purpose of reducing the power and improving the propulsive efficiency. The two-dimensional two-degree-of-freedom bionic flap equations of motion were also established, and the flap simulation was carried out by using Ansys Fluent, and the relationship curves between the propulsive efficiency and Strahl's number (St) were plotted, and the optimal working conditions in the simulation were obtained by the optimal propulsive efficiency on the relationship curves. The results show that the optimum propulsive efficiency is 28% under the operating conditions of A = 0.2 m, f = 1.2 Hz and v = 0.8 m/s. Finally, a manta-like robot model was fabricated in this paper, and the conclusions were verified by pool tests.