In the theory research and engineering practice, more basic inflatable models are essential for the mechanical property analysis of inflatable structures. Firstly, this paper presents a model of the tapered inflatable cantilever beam based on Timoshenko's theory and analyzes its deformation under a concentrated force. Moreover, the following forces resulting from internal pressure and taper ratio are introduced into the equilibrium equations of the deformed configuration. Thus, the model is optimized compared to the existing one for a straight beam. To verify the effectiveness and the superiority of the established model, the theoretical method based on the model and FEM method are compared by adopting an example about the tapered beams. Finally, the theoretical method is applied in analyzing the influence of geometry and estimating a valid range of taper ratio. By the criterion of the same amount material area, the optimum taper ratio is obtained.