This paper focuses on realistically predicting the power outputs of wave energy converters operating in shallow water nonlinear waves. An oscillating surge wave energy converter (OSWEC) is utilized as a specific calculation example, and the generated power of the OSWEC has been predicted by using a new method (a nonlinear simulation method) that incorporates a second order random wave model into a nonlinear dynamic filter. It is demonstrated that the second order random wave model in this article can be utilized to generate irregular waves with realistic crest-trough asymmetries, and when used in combination with the nonlinear filter can produce more accurate power output predictions. The research findings demonstrate that the new nonlinear simulation method in this article can be utilized as a robust tool for ocean engineers in their design, analysis and optimization of wave energy converters. (C) 2017 Elsevier Ltd. All rights reserved.