This paper is concerned with the parameter optimization for a simplified propulsive model of biomimetic robot fish propelled by a multiple linked mechanism. Taking account of both theoretic hydrodynamic issues and practical problems in engineering realization, the optimal link-length ratio is numerically calculated by an improved constrained cyclic variable method. The result is successfully applied to the 4-linked robot fish developed in our laboratory. The comparative experiments on forward swimming speed of the robot fish before and after parameter optimization verify the effectiveness of our method.