In earlier designs of biomimetic robot fish, they were divided into a mechanism system and a control system, with the emphasis placed on realizing the functional target. However, subsequent maintenance and repair in the design procedure were never considered. Hence, this study will adopt the module concept to develop both the mechanism system and the control system. The modular design aims to make the actuator watertight, the sensor watertight and the control circuit watertight, in that order, and to combine the actuator and mechanism into a single oscillatory mechanism module. The entire oscillatory mechanism consists of three sets of modularized oscillatory mechanisms; the control module is a control box, composed of the control circuit, the power, and the floating/diving mechanism. The sensor undergoes a separate watertight treatment to reserve the condition of the interface with the control circuit. It is designed with a simple buoyancy tank to adjust the buoyancy and stability.