This paper proposed a coupling relationship between the substrate and piezoelectric sheet, and a nonlinear model of bimorph actuator. The coupling relationship and analytical nonlinear model are based on the theory of composite material, Euler—Bernoulli beam, and nonlinear constitutive equation of piezoelectric material, combining with the research on the distribution of stress and strain of actuator. The analytical nonlinear model is simpler, with concise physical meanings, and result of the model is consistent with that of experiment. Then we analyze the characteristics of piezoelectric devise of different structure parameters. By the analytical relationship, the influence of structure parameters on the performance of piezoelectric actuator can be indicated clearly.