Wave glider is an unmanned surface vehicle which can directly convert wave energy into forward propulsion and fulfill long-term marine monitoring. Previous study suggested that the wave motion and stiffness of restoring springs mounted on the hydrofoil are main factors affecting the propulsion performance of wave glider. In this paper, the dynamic responses and nonlinear characteristics of underwater propulsion mechanism considering the nonlinear stiffness of restoring springs are investigated based on a fluid-rigid body coupled model. Firstly, the models of propulsion mechanism with different kind of restoring spring are proposed, and the linear and nonlinear characteristics of restoring spring are considered. Then, a fluid-rigid body coupled model of wave glider is developed by coupling the rigid body dynamics model and hydrodynamic model. Dynamic responses are simulated by numerical analysis method and the nonlinear characteristics with different restoring springs are illustrated by time/frequency domain motion response and phase diagram analysis. The effects of wave excitation frequency and wave heights on the propulsion performance of wave glider are analyzed. The results show that, multi-frequency responses occurred in propulsion system. And the study suggests that the nonlinear restoring spring on the hydrofoil can be suitable for different sea condition and better propulsion performance can obtained than linear stiffness spring, which provides a reference for developing propulsion mechanism with high performance in complex marine environment.