The invention discloses a method for preparing polyacrylamide, belonging to the technical field of synthesis of water-soluble macromolecular materials. According to the method, polyacrylamide with the molecular weight being over 1 million can be prepared by adopting inorganic nano-silica and ammonium persulfate as a compound initiator, carrying out low-temperature and low-pressure reaction, then drying and crushing. Due to the compound initiator, acrylamide is promoted to be polymerized on the surfaces of nanoparticles; compared with the traditional preparation methods, the method has the advantages that the preparation process is simple, the conditions are easily controlled, the large-scale industrial production is facilitated, and the molecular weight of a product is about 1.7 million.