WaveRoller can be classified as an effective Oscillating wave surge converter (OWSC) installed in nearshore coastal areas. In this study, the hydrodynamic performance of a two-dimensional (2-D) WaveRoller is investigated based on a fully nonlinear time-domain higher-order boundary element method (HOBEM). The mechanism through which the wave power is extracted, is analyzed in a nonlinear power take-off (PTO) by coupling the time-varying motion of the flap and hydro-electric generator. A simplified Morison model is used to determine the viscosity term in the structural motion equation. The present model is validated against the published experimental and numerical results for an OWSC with its top edge piercing through the water surface. Numerical simulations are undertaken to investigate the mechanism of the nonlinear phenomenon including wave nonlinearity and PTO nonlinearity. The influence of the viscosity is also quantified by comparing the numerical results with and without the quadratic damping term. Numerical results indicate that when the incident wave amplitude is large, the capture efficiency is reduced by the strengthen higher order free wave components, and the PTO nonlinearity becomes more prominent. Furthermore, the maximum discrepancy induced by the viscosity occurs in the resonant zone.