A long uncompressed fin driven by fin rays is designed in this paper, and its kinematic model is established. The SST K-ω turbulence model is used to solve the N-S equation of three-dimensional unsteady incompressible fluid, and the hydrodynamic characteristics of the uncompressed fin are studied. The results show that when the undulating fin is started, the velocity of the surrounding fluid gradually increases with the movement of the fin surface, and the fluid moves from the leading edge of the undulating fin to the back edge along the fin surface. A relatively obvious anti-Karman vortex street is formed on both sides of the fin, and a jet is formed at the tail of the fin to generate forward thrust.