The characteristic impedance of GTEM cell is analyzed by three-dimension finite element method.The calculation model is the sect cut from the GTEM cell along the spherical wave direction.The optimized angle of septum is found through calculating the characteristic impedances for different sects and different angles.The optimum plan for correcting the septum is proposed,considering the effect of the thickness of septum.The(user's) program module for calculating the characteristic impedance is developed under the environment of ANSYS,which greatly improves the design efficiency.The method discussed in this paper approximate,perfectly to the real state of electromagnetic field distribution,so it's more accurate.