Abstract
A Cylindrical Gas Electron Multiplier (CGEM) is under construction for the upgrade of the BESIII inner tracker. A preliminary simulation of the CGEM signal before the frond-end electronics has been worked out by making some sampling models from the beforehand Garfield++ simulation. The electronics response functions are implemented making the simulation of CGEM more complete. By comparing the cosmic-ray data acquired by two layers of CGEM and the simulation, the micro-sector effect is simulated and some key parameters in the sampling models are fine tuned in this order: gain, charge sharing, charge fluctuation and electron diffusion. A general agreement is achieved between the cosmic-ray data and the simulation with these improvements.