The invention discloses a method for synthesizing high molecular weight instant anionic polyacrylamide, which is characterized in that: an alkaline hydrolysis method is adopted, and comprises the following steps of: preparing aqueous solution of acrylamide monomer from the acrylamide monomer by using deionized water, adding a hydrolytic agent, a composite initiator and an auxiliary agent, regulating the pH value to be between 8.5 and 12, introducing nitrogen, expelling oxygen and sealing a system at the temperature of between 10 and 25 DEG C, polymerizing under the heat insulation condition, and hydrolyzing, crushing, drying and grinding to obtain instant anionic polyacrylamide solid powder with the viscosity-average molecular weight of 15,000,000 to 21,000,000 and the dissolution time of shorter than 0.5 hour. A novel coordination agent and a novel pH value regulator are adopted, the method is easy and convenient to operate, the initiation speed of the monomer is high, the dissolution speed of a product is high, the reaction rate and the molecular weight of the product are controllable, the method is suitable for production on different scales, and the product is suitable for industries such as papermaking dispersants, tertiary oil recovery, beneficiation, sewage treatment and the like.