A technique for preparing polyaluminium chloride is characterized in that: the technique controls the water insoluble substances by a two-step coagulating sedimentation method and comprises the steps of: (1)generating aluminic acid solution by the reaction of aluminium-containing raw material and hydrochloric acid in an acid-proof vessel, adding acid soluble coagulant to execute coagulating sedimentation, and then picking up the supernatant as mother liquor; (2)adding water into the mother liquor obtained in step (1), and heating up in an acid-proof vessel, and then adding calcium aluminate to react in atmospheric pressure, finally adding polymerization coagulant; (3)precipitating to obtain the supernatant which is liquid PAC, obtaining solid PAC by means of drying. The present invention has the advantages of that: the two-step coagulating sedimentation method can overcome the problem of that the control and reduction of the water insoluble substances in PAC preparing process, which uses calcium aluminate as raw material, is hard, the operation is simple, the cost is lower, the settling time can be reduced at 10~20 times, the operability is better, the economic benefit is obviously and each index of the PAC product can satisfy the national standard.