Wave-driven unmanned surface vehicle (WUSV) is a new concept marine robot drived by wave energy and solar energy, very suitable for the observing vast oceans with incomparable endurance. Firstly, the multibody system of WUSV is described based on D-H approach and the velocity and position of each moving part in WUSV are represented. Then, the driving principle is analyzed and the WUSV dynamic model in three dimension is established by Lagrangian mechanics. Finally, we perform the motion simulation of WUSV under certain sea conditions. These tasks provide the theoretical foundation for further study on the WUSV motion control and efficiency analysis.