Abstract: On the basis of the slender body theory of James Lighthill, this paper calculates the thrust efficiency of robotic fish with the law of conservation of energy, the law of momentum and the law of wave. The equation of thrust efficiency with the parameterized spring coefficient is established and the equation is simulated. Simultion results show that the efficiency of large robotic fish can be increased greatly by adding spring. It may be a new idea for the research of large scale autonomous underwater vehicle.