The invention provides a method for preparing cationic polyacrylamide. The method mainly utilizes the reducibility of amine impurities in a domestic cationic monomer, a small amount of oxygen gas dissolved in a system is used as an oxidant, aids such as sodium bisulfate with relatively high reaction activity on the oxygen gas are added to form a redox initiator for initiating prepolymerization of the system, after the monomer impurities with functions of chain transfer and polymerization inhibition and the oxygen gas in the system are used up, further a proper amount of an initiator are added, and the obtained material is transferred into polymerization equipment to be polymerized. The method has low requirement on the purity of the cationic monomer, the oxygen does not need to be removed, and in addition, polymerization is performed in sections, so that convenience in heat radiation control can be achieved, and relatively high practical values of industrial production can be made. In addition, by adopting the method, dependency on an imported low-impurity cationic monomer is overcome, the production cost is lowered, and the method is particularly applicable to preparation of an acrylamide copolymer of a high cationic degree and/or a high molecular weight.