The dynamics of a wave power generation system which consists of wire(s), float(s), counterweight(s) and ratchet mechanism is examined in this paper. The model is composed of the several equations: the equation for the generator, the equation for the float and counterweight motion, the equation for the driving pulley motion. The hydrodynamic excitation force is calculated numerically along with the added mass force and damping force, which are then combined with the equation for float motion given by the rigid body dynamics to obtain the float displacement and other physical quantities. Wave tank test results are found to agree closely with the numerical results, thereby validating the proposed numerical model. Key physical quantities, electrical power output and maximum wire tension, are examined for different wave conditions.