The inner drift chamber of the BESIII experiment is encountering an aging problem after running of several years. A Cylindrical Gas Electron Multiplier Inner Tracker (CGEM-IT) has been an important candidate for the upgrade of the inner drift chamber. In order to understand the specific detection behavior of CGEM-IT and to build a digitization model for it, a detailed simulation study with the Garfield++ program has been performed. Distributions related to the Lorentz angle, the diffusion effect, the drift time, the multiplication and the signal induction on the readouts are obtained from this simulation. Based on these studies, a preliminary digitization model is implemented in the BESIII Offline Software System and the preliminary result is reasonable.