This paper aims at designing a biomimetic robot fish featured with an undulatory propulsion system, and divides the implementation procedure into mechanism system and control system. In the mechanism system, four modes, namely, moving forward, turning right, turning left, and floating and diving are designed. These motions are based on the observation of a real fish, wherein each type of motion is analyzed. The control system is composed of a control chip, actuator, and sensor. The controller sends a signal to the actuator, which then drives the mechanism to make the oscillatory motion, and the sensor will inspect any obstruction using the infrared ray. Through this system, the biomimetic robot fish could evade any obstacle. The biomimetic robot is wrapped with a waterproof jacket that protects the internal electrical circuit and mechanism.