This paper describes a design of soft pneumatic actuator(SPA), the fabrication process of which was based on 3D print technology. Optimal design was done by using non-linear finite element method, in which geometrical and material non-linearity were considered. In this paper, the characteristics of two kinds of SPAs, single material actuator(SMA) and dual material actuator(DMA) were analyzed and evaluated at the same time to lead to optimal actuator designs, which were validated against experimental data of one-degree bending. In order to get the largest deformation of the actuator under given pressure, the structure of SMA and DMA were studied. In the end, the optimal actuator design was applied to make a gripper which was combined of three DMAs.