Soft pneumatic actuators(SPAs) can be adapted to grasp objects of different shapes, and it is helpful to estimate the SPA's deformation under external force, but their low stiffness bring signifcant challenges to the experimental characterization and modeling of their statics. Aiming at achieving the target of estimating SPA's deformation through establishing relation models between external force, air pressure, and SPA's deformation, a practical and straightforward estimation method is proposed in this paper. The nonlinear Euler-Bernoulli beam(NEBB) is applied, the SPA is simplified as a nonlinear cantilever beam, and the complex internal air pressure effect is equivalent to the external moment to analyze the SPA's deformation. The proposed nonlinear beam SPA method and finite element method(FEA) are compared with the experimental data, respectively, and the estimated deformation curves of proposed nonlinear beam SPA method agree well with the experimental results than FEA. It is shown that SPA's deformation can be estimated accurately through the proposed method. The study establishes a nonlinear beam SPA method baesd on nonlinear Euler-Bernoulli beam to estimate the SPA's deformation under external force and can potentially be used to analyze various SPAs’s mechanical property.