To enhance the propulsive speed, reduce the control complexity, and improve the system reliability of bionic underwater robot, a cam-driven high-frequency undulating fin is proposed in this paper. The oscillation motion of every fin ray is achieved by a type of cam-rocker mechanism; and a series of fin rays which are arranged parallelly have a fixed phase difference by mechanical regulator, in which case only one motor is necessary for wave propagation of the flexible fin. The structure of the bio-inspired undulating fin is designed and the transmission motion is analysed. A free-swimming underwater robot which is propelled by one mechanical fin is proposed. Computational fluid dynamic simulation is conducted to explore the propulsive force and velocity of the undulating fin at high frequencies. Finally, the robot prototype is developed and the propulsive velocity is tested. The simulation and experiment results verify the feasibility of the high-speed undulating fin and reveal that the forward velocity can be significantly improved by increasing undulating frequency.