Scintillator and opto-electronic conversion device coupled X-ray detector is widely used in industry CT system. The scintillator of X-ray detector converts X-ray into visible light. The scintillator as a major part of detector affects the imaging efficiency and resolution of the detector. Based on the Geant4 simulation platform,the Monte Carlo model was built to simulate the characteristics of Cs I（ Tl） scintillator under different scintillator thicknesses and different X-ray energy（ 20—80 ke V）. Furthermore,the effects of scintillator conversion efficiency and resolution with different scintillator thicknesses were calculated.Simulation results show the ideal thickness of the scintillator for the given input energy. The simulation results provide a reference for the parameter design and optimization of the scintillator.