Polysilicate aluminum chloride (PASC) flocculant was prepared with a coal gangue for the resource utilization of coal gangue. The effects of alkali cement ratio, roasting temperature, acid dosage, acid leaching temperature, liquid to solid ratio and acid leaching time on the performance of PASC were investigated. The physicochemical properties and microstructure of PASC were analyzed and the flocculation mechanism of PASC was discussed. The results show that the turbidity removal rate of PASC to coal slime water can reach 99.08% under the optimum conditions (i.e., alkali cement ratio of 0.9, roasting temperature of 900 ℃, acid dosage of 1.75 g/g, acid leaching temperature of 49 ℃, liquid to solid ratio of 8.9, and acid leaching time of 2.8 h). Since PASC is an amorphous polymer formed by the complexation of polysilicic acid, aluminum ions and hydroxyl groups, it has a rich spatial network structure and its flocculation mechanism includes adsorption bridging, trapping and sweeping, and charge neutralization.