When we prepare glass ceramics from iron tailings, the residual iron oxides has a great influence on the crystallization process, crystalline phase and microstructure of glass ceramics. The mechanism of iron oxides in the preparation of glass ceramics was studied by DSC, XRD, SEM and Mossbauer spectrum. Results show that Fe2+ is mainly in the octahedral coordination structure which may promote glass phase separation, while Fe3+ is partly in the tetrahedral coordination structure which may strengthen the glass network connection and partly in octahedral coordination structure similar as Fe2+. With the increase of total iron content, the microstructure of glass ceramics changes from glass to columnar crystal, then to granular crystal, and finally to small globular grains. The main crystal phase of glass ceramics changes from cordierite to Mg-Al spinel. In order to obtain the better compactness of glass ceramics, the octahedral coordination structure of Fe2+and Fe3+should be appropriately increased.