Oscillating water column (OWC) device is one of the most promising wave energy converters (WECs). Besides the energy conversion efficiency, the survivability should also be considered for a design purpose in the process of wave energy exploitation. In the present study, by introducing the artificial viscous terms into the dynamic free surface boundary condition and Bernoulli equation, a fully nonlinear numerical model based on higher-order boundary element method (HOBEM) is adopted to model the wave dynamics of an OWC device. The viscosity effects on the wave force (i.e., ΔF) is investigated by comparing the predicted wave force by the numerical model with and without these viscous terms. The effects of the chamber geometry parameters, such as front wall draft, chamber width and opening ratio (i.e., air orifice width), on ΔF are investigated. The results indicate that the viscosity effect on the wave force on the seaside surface of the front wall is larger than that on its shoreside surface. The viscosity effect on the total horizontal wave force on the front wall increases with the increase of front wall draft in some extent. The influence of the viscosity on the horizontal wave force increases with opening ratio decreasing due to the increasing air pressure inside the chamber.