Abstract:GNSS radio occultation technique has been proved to be an important method to investigate the ionospheric electron density in different levels.3D ray tracing is the best choice for analyzing the propagation of ray in ionosphere.A group of matrix equations from the differential equation in Cartesian coordinate was used to analyze the detail steps of ray tracing with the formulas.The NeQuick model is used to simulate the excess phase of L1,L2 and L5,and the simulated results are used to retrieval the electric density profile with absolute TEC inversion method with different band combination.The result of two numerical experiments shows that the combination L1-L5 is the preferable choice for vertical density profiles inversion under the identical observation conditions.