Abstract: Biological studies show that the stiffness of fish body has a significant impact on their propulsive performance, and the excellent swimming abilities can be achieved by varying body stiffness. However, the complex relations among the muscle actuations, the body stiffness and the swimming performance are still unclear. Therefore, the body of anguilliform fish, taken as the research object, is modelled by a viscoelastic beam in present study, and the Taylor resistance fluid theory is adopted to establish swimming fish model, in which the kinetic model of calcium ions is used to model the muscle actuations. The effects of muscle actuation and the variable body stiffness on the propulsive performance are analyzed. The results showed that with the increase of body stiffness, the force of muscle actuation was increased rapidly and then decreased slowly. When the force of muscle actuation increased, the forward speed increased with the body stiffness, and then phoned to stable gradually. When the frequency of muscle actuation was increased from 1 Hz to 2 Hz, the swimming speed and the start acceleration can be increased by 55% and 129%, respectively. These results indicated that the propulsion performance of swimming fish can be significantly improved by increasing the body stiffness. To verify these conclusions, an experiment platform, based on the series-parallel mechanism with variable stiffness, was proposed in present study, and the results also found that the variable stiffness of fish body had a significantly influence on the propulsive performance. In the experiments, the thrust can be increased by 2.5 times when the driving frequency of the servo was increased from 1 Hz to 2 Hz, and the spring stiffness was uniformly changed. Overall, these results throw a light to design robotic fish with better swimming performance by changing the body stiffness, and provide a theoretical basis for developing a robotic fish with variable stiffness.