Abstract: Calcium aluminate clinkers, in which the mass ratio of Al₂O₃ to SiO₂(A/S) is 5, were prepared at 1500℃ for 1h by proportioning pure reagents. The alumina leaching properties and crystal structure of 12CaO·7Al₂O₃ in the clinkers with different molar ratios of CaO unbonded with SiO₂ to Al₂O₃ (C/A) were studied. The effect mechanism of C/A on the clinkers was also discussed by X-ray diffraction and X-ray fluorescence analysis. When C/A is 1.0 to 1.4, the alumina-leaching ratio improves because the amount of 12CaO·7Al₂O₃ increases with increasing C/A. When C/A is 1.4, the alumina-leaching ratio reaches the maximum value of 93.30%. In this case, 12CaO·7Al₂O₃ with defects forms, and the amount of substance of CaO is less than 11.3 mol. With the further increase of C/A, the alumina-leaching ratio decreases, which is because the preferred orientation of 12CaO·7Al₂O₃ changes from the (211) crystal plane into the (420) crystal plane.