A refined ionization model of small molecules was developed based on a unit of classic models. The ionization process was divided into three sub-processes of diffusion, charge transfer, and ion evaporation. Based on the equations, the impact of a single process and multiprocesses on response was verified by employing special samples, and the relative evaporation energies of several pair of samples were calculated. This model is sensitive to slight differences in ionization energy and allows distinction of the ionization differences during each sub-process.