For improving fast-start performance of underwater propeller, the robot fish with high maneuverability is developed using the fish-like swimming mode characterized by undulatory flexible body. Through observations of C-start in crucian carp, the kinematic model of fast-start in robot fish is set up. According to hydrodynamic analysis on unsteady effect of added mass and wake vortices, liquid reaction forces on bionic fish-body are derived. Based on simulation experiment of C-start, propulsive performance indexes including kinetic energy, power and propulsion efficiency are presented respectively. The research results can provide theoretical basis of raising propulsive efficiency of C-start in subsequent underwater experiments, and optimization design of robot fish.