Compared with traditional submersibles, bio-mechanism based robotic fish has higher submerged maneuverability and propulsion efficiency. In order to maximize the thrust power of robot fish, a technique using dynamic meshes as well as User defined function (UDF) to control moving boundaries is established to simulate the circumference fluid of a swimming fish body, as well as a fish robot. The resistance of fluid and propulsion power is analyzed numerically under various conditions, and we try to clarify the main factors to influence the swim speed of the two models from their velocity and pressure fields.