Abstract: With the test model of bionic underwater vehicle based onundulatory propulsion of long flexible fin as the background, the paperstudies the undulatory propulsion dynamics of the longflexible fin of Gymnarchus Niloticus. Assuming that the fin bonevibrates according to a sinelaw, this paper establishes the mathematical model of undulatory curvedsurface of the long flexible fin. Then using the resistive theoryand a modified hypothesis about fluid, the propulsion force generatedby undulation of the long flexible fin is analyzed, and the dynamic model is obtained. Finally, with the aid of numerical simulation, thepaper obtains the formula for hydrodynamic force and moment of the bionic longflexible fin of the test model caused by the fin undulatory movement. This formulaprovides a theoretical basis for dynamic analysis and control system design ofbionic underwater vehicle.