It is well known that the fiber core refractive index control in fiber fabrication is important, especially for erbium doped fiber (EDF). The refractive index will affect the cutoff wavelength of EDF, and then will bring much impact on the gain performance of EDF. In this paper, the refractive index was measured and the composition of several EDFs was analyzed by electronic probe microbeam analysis (EPMA). The EDFs were fabricated through modified chemical vapor deposition (MCVD) method combining with solution doping technology. It was found that the aluminum codoping would seriously affect the ratio of silicon to germanium in the core, the more aluminum incorporated into the porous layer, the less germanium would stay in the EDF. It was explained with the two chemistry equations of alumina creation and germanium oxide volatilization. Codoping aluminum and germanium into silicate both would increase the refractive index, but index decreasing brought by the amount of germanium decreasing was larger than the index increasing brought by the amount of aluminum increasing, which led to the refractive index decreasing along with the amount of aluminum increasing.