A stochastic averaging method is proposed for nonlinear piezoelectric energy harvesting subject to Gaussian white noise excitation.The Fokker-Planck-Kolmogorov equation of the electromechanical coupling system is a three variables nonlinear parabolic partial differential equation whose exact stationary solutions are generally hard to seek.In order to overcome the difficulties in solving the partial differential equations, the van der Pol transformation scheme is applied to decouple the electromechanical coupling equations.