Motions of natural creatures appear flexible and rhythmical characteristics. The bionic undulating propeller imitates undulating motion of a fish’s flexible long fin, and its propelling efficiencies are affected by bionic units’ characteristics. In order to improve the flexibilities of both structure and bionic motion, the paper designed a bionic undulating propeller driven by hydraulic system, and then carried out some modeling work on the rotatable direction valve. At last, some experiments are brought forward on a prototype of bionic undulating propeller. In the experiments, rotatable-direction-valve could control fluid flowing through all the oil orifices periodically and in sequence, so as to drive the bionic units swinging rhythmically. Under the control of rotatable-direction-valve, the prototype gains flexible starting and stopping processes, as well as self-resume character of bionic undulating wave, which are coincident to nature fish. The results show that application of hydraulic system in bionic undulating propeller is feasible, and theoretic model of rotatable direction valve is correct.