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 950-1200 ℃ . C₂AS is mainly produced by the reaction of CA and CaO decomposed from C₄A₃＄ with the residual SiO₂ when the temperature exceeds 1200 ℃ . It is indicated that the kinetics of the C₂AS formation above 1200 ℃ 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.