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As mining waste, alunite is a potential resource to produce potassium salt. The decomposition of alunite is closely associated with the recovery of soluble potassium. In this study, the effect of CaO on phase transformation of alunite in the desulfation stage was examined. The results showed that CaO was beneficial to the desulfation of alunite. The decomposition temperature to obtain soluble potassium salt (K2SO4) was reduced from 800 °C to 700 °C by adding CaO. When the mass ratio of CaO/alunite was 0.1, 81% of soluble potassium was extracted by water leaching after calcination at 700 °C for 2 h. The mechanism of CaO to promote the disintegration of alunite was proposed through analyzing the phase transformation sequences. Alkaline Ca ion was inclined to bond with acidic [SO4] groups, and thus the breakage of S–O linkages between [AlO6] octahedron and [SO4] tetrahedron were improved. Monomer [SO4] tetrahedrons were released to form K2SO4 at a lower decomposition temperature. With the increase of the amount of CaO, the excess CaO bonded with neutral Al. [AlO6] tetrahedrons in alunite transformed into [AlO4] octahedrons due to the breakage of the Al–O network. Al3+ was dissociated and bonded with [SO4] tetrahedron to form soluble Al salts.