This paper develops a bionic propulsor for underwater propulsion by mimicking the flapping motion of a sea turtle's forelimb. A two-degree-of-freedom propulsion model was created to coordinate pitching and flapping movements. The hydrodynamic model of the turtle forelimb was obtained using computational fluid dynamics (CFD) simulations with the User-Defined Functions (UDF). The study analyzed the independence of the mesh and underwater propulsion performance, examining the effects of flapping frequency, flapping angle, and pitching angle on thrust. Results indicate that the flapping frequency significantly affects propulsion; increasing the frequency from 1 Hz to 2 Hz resulted in a fivefold improvement in performance. Additionally, thrust increased with pitch angle, whereas higher flapping angles also boosted thrust, but the effect diminished over time. The study identified optimal parameter values by comparing the simulation data and investigated the effects of three-dimensional vortices and surface pressure dynamics on propulsion. These findings offer valuable insights for the design of bionic propulsion systems and advancing underwater propulsion research inspired by sea turtles.