In this paper, we prepare several GexSb20Se80-x glasses (x=5 mol%, 10 mol%, 15 mol%, 17.5 mol%, 20 mol%, and 25 mol%), and measure their Raman and X-ray photoelectron spectra (Ge 3d, Sb 4d, and Se 3d) in order to understand the evolution of the glass structure with chemical composition. We further decompose the spectra into different structural units according to the assignments of these structural units in the previous literature. It is found that the structural units of SeSeSe trimers exist in the Se-rich glasses, but the number of the structural units of trimers decreases rapidly with the increase of Ge concentration and finally becomes zero in Ge15Sb20Se65 glass. With the increase of Ge concentration, the quantity of GeSe4/2 tetrahedral structures increases, but the number of SbSe3/2 pyramidal structures remains almost unchanged in the Se-rich glasses. On the other hand, the numbers of GeGe and SbSb homopolar bonds increase with the increase of Ge concentration, but those of the GeSe4/2 tetrahedral and SbSe3/2 pyramidal structures decrease in the Se-poor glasses. Moreover, the SeSe homopolar bonds exist in all the glasses, and they cannot be completely suppressed. When the composition is close to stochiometric value, the glass is dominated by heteropolar GeSe and SbSe bonds, but has negligible quantities of GeGe, SbSb and SeSe homopolar bonds. The transition threshold, rather than the transition predicted by the topological constraint model, occurs at the chemically stoichiometric glasses. This suggests that chemical order, rather than topological order, is a main factor in determining structures and physical properties of GeSbSe glasses.