Taking the twodegreeoffreedom Lbeam bistable piezoelectric energy harvester as the research object, the effect of the firstorder and secondorder nonlinear coefficients in the nonlinear piezoelectric coupling relation on the output power of the energy harvester and the motion of the main beam is studied. The results of numerical simulation for the dimensionless dynamic equation of the energy harvester show that the nonlinear piezoelectric coupling has obvious influence on the output power of the energy harvester and the motion of the main beam when the excitation amplitude is large, the larger the nonlinear piezoelectric coupling coefficient, the larger the output power, and the smaller the vibration displacement of the harvester. When the firstorder nonlinear coefficient is positive, the response of the harvester with nonlinear piezoelectric coupling is better than that of the harvester with linear piezoelectric coupling, and quite the opposite when firstorder nonlinear coefficient is negative. However, no matter what values the secondorder nonlinear coefficient will take, the response of the harvester with nonlinear piezoelectric coupling is better than that of the harvester with the linear piezoelectric coupling.