As a kind of two-dimensional materials,graphene has many excellent characteristics,such as zero band-gap,high electron mobility,high thermal conductivity and high mechanical strength,which make it widely applied in the field of photoelectric detection.X-rays can be detected based on the electric field effect of graphene.When the detector working,electron-hole pairs are generated in the substrate by the incident X-rays.The drift of the electrons in the substrate will change output of the detector.Therefore,it is necessary to study the transportation of electrons in the graphene-based X-ray detector.The motion of electrons in the detector is simulated using finite element method,and the influence of the electrons’ motion to the detector output is analyzed.Dependence of the detector output on the incident position of X-rays is studied.Also,the influence of structure parameters to the performance of the detector is analyzed.Based on the results of this study,performance of the graphene-based X-ray detector can be optimized.