A simplified model of the thrust force is proposed based on a caudal fin oscillation of an underwater bionic robot. The caudal fin oscillation is generalized by central pattern generators(CPGs). In this model, the drag coefficient and lift coefficient are the two critical parameters which are obtained by the digital particle image velocimetry(DPIV) and the force transducer experiment. Numerical simulation and physical experiments have been performed to verify this dynamic model. A simplified model of the thrust force is proposed based on a caudal fin oscillation of an underwater bionic robot. The caudal fin oscillation is generalized by central pattern generators (CPGs). In this model, the drag coefficient and lift coefficient are the two critical parameters which are obtained by the digital particle image velocimetry (DPIV) and the force transducer experiment. Numerical simulation and physical experiments have been performed to verify this dynamic model.