Based on the principle of 2-dofs inertia pendulum, a wave energy converting device (WECD) to realize the energy self-supply for underwater vehicles was proposed in the paper, which can absorb movements or forces from two directions perpendicularly. The key part is the one-way device, which converts the irregular oscillation into one-way rotation. Therefore, the energy obtained is stored in torsional coil spring to drive a generator. The dynamic equation of the simplified model was presented on the basis of Lagrange equation, and the variations of angles in 2-dofs were obtained through an example by Runge-Kutta (RK), and the parameters and qualifications for the device optimization were also analyzed by comparison. The analysis provides necessary information for dynamic performance analysis, optimal design, efficiency analysis and control of WECD.