Refraction is the main characteristic when electromagnetic wave propagates through ionosphere,which is mainly determined by the free electrons in ionosphere.In this paper the relationship between the refractive index(n),the electron concentration(N_e) and the altitude(h) is analyzed,and numerical simulations are taken on the refractive index and the electron concentration relating to the altitude.We contrast the simulated altitudinal profile of critical frequency and the actual altitudinal profile of critical frequency,and the results show that at high ionosphere,the simulations are roughly in agreement with the actual measurements,while at the lower ionosphere the simulations deviate from the measurements by reason of nonlinear relationship and existence of E_s layer.