Compared to rigid robotic actuators, soft actuators based on fiber-reinforced elastomeric actuators have great potential for use in exploring complex terrain, medical operations, and flexible capture of targets. However, it is still challenging to model the mechanical behavior of soft actuators because of the large deformation and nonlinear characteristics of elastic materials. This article proposes an analytical model to predict the shape of a soft actuator to better describe the relationship between its deformation and input pressure, gravity, and other external forces. We first derive the relationship between soft actuator deformation and pressure using volume maximization and the principle of virtual work. We further use Euler–Bernoulli beam theory to investigate the influence of self-gravity and external forces on the soft actuator configuration. The model is verified by fabricating a soft actuator prototype via mold casting. Finally, we perform a series of experiments to evaluate the accuracy of our proposed model. The longest total time to solve the model is 0.035 s. Experimental results show that the model's maximum error rate is between 2.89% and 9.75%. This indicates that the model can effectively predicts the deformation of soft actuators considering gravity and other external forces.