The photoelectric conversion efficiency of organic materials is limited by the exciton splitting at the organic interface which is determined by the exciton binding energy (E-b). However, E-b is controversial and hard to directly measure by current methods. We propose a dispersion relation to establish a model to accurately extract E-b from absorption spectrum. We synthesize an organic-inorganic hybrid perovskite crystal (CH3NH3PbI3) and use the absorption spectrum to verify the method. The result of E-b is 15 meV at room temperature, which is smaller than 26 meV (thermal energy), implying that our model is not affected by the hot carrier effects. Moreover, this model is applicable to various organic materials (18 kinds). Especially, the binding energy of perovskite is 5 meV under 2 K, which is the same as the result estimated experimentally under 50 Tesla condition. Furthermore, this model allows estimating the effective mass, dielectric constant and excition radius. This work will allow a better understanding of the optoelectronic properties at the interface of organic materials.