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The bionic undulating propulsion method has the advantages of good manoeuvrability, strong adaptability, and environmental compatibility, which shows broad prospects in the application of underwater robots. To enhance the propulsion speed, reduce the weight and volume, and improve the reliability of the bionic undulating fin, a modular bionic undulating propeller based on cam mechanism was proposed. Active disturbance rejection control (ADRC) method was used to achieve the continuous accurate control of wave frequency. The structure of the undulating fin was designed. The mathematical model of the sinusoidal wave generating mechanism was established. The instantaneous efficiency formula of the cam mechanism was derived, resulting in an average efficiency of 83.6%. The motion simulation was carried out and the result proved the feasibility and the correctness of the cam parameters calculation. Taking the output frequency as the control target, the control model of the undulating fin was established. The theoretical derivation and computational fluid dynamics simulation analysis showed that both the internal characteristics and the external loads of the undulating fin suffered from nonlinear time-varying effects at constant frequency. To overcome the negative effect of internal and external disturbances, the linear active disturbance rejection controller was designed for frequency control. Based on STM32 single chip microcomputer, the experimental measurement and control system of the undulating fin was realized. The experimental results showed that the undulating fin could accurately track the expected frequency at both low and high frequencies. The response curve was smooth and continuous without overshoot, and the steady-state fluctuation error was less than 2.3%. When the frequency was 3Hz, the output fluctuation errors of the active disturbance rejection controller and the proportion integration differentiation controller were 6.3% and 2.1%, respectively, and the control accuracy was improved by 66.7%. The research results showed that the modular biomimetic undulating fin propeller had satisfactory control precision and good integration. As it can be configured to biomimetic underwater vehicles in any numbers, the modular undulating fin had good application value.