To reveal the mechanism of the inert gehlenite (C2AS) phase in sulfoaluminate cement clinker at a low Al2O3/SiO2 ratio, a series of clinkers were prepared at different calcination temperatures and holding time as well as a fixed Al2O3/SiO2 ratio of 2. The evolution of C2AS was investigated by X-ray diffraction combined with the Rietveld method and thermogravimetry-differential scanning calorimetry. The results show that C2AS is produced by the reaction of CaO, SiO2 and Al2O3 at 950-1 200 ℃. C2AS is mainly produced by the reaction of CA and CaO decomposed from C4A3$ with the residual SiO2 when the temperature exceeds 1 200 ℃. It is indicated that the kinetics of the C2AS formation above 1 200 ℃ is mainly controlled by the diffusion of Ca2+, and the activation energy of the reaction is (305±20) kJ/mol. This study provides a a guidance for the use of low-grade bauxite in the calcination of sulfoaluminate cement clinker.