The invention relates to the field of sizing agents, in particular to an alkenyl succinic anhydride sizing agent. The sizing agent contains sodium carboxymethylcellulose, urea and alkenyl succinic anhydride. Preferably, the weight ratio of the sodium carboxymethylcellulose to the urea to the alkenyl succinic anhydride is (2.8-7): (4-12.5): 100. The sizing agent is obtained by the following steps of: dissolving the sodium carboxymethylcellulose and the urea into deionized water to form liquid A, regulating the pH value to be between 6 and 7, mixing the liquid A and the alkenyl succinic anhydride, and stirring uniformly to obtain the emulsion alkenyl succinic anhydride sizing agent. The weight ratio of the liquid A to the alkenyl succinic anhydride is (3-9): 1. The consumption of the sodiumcarboxymethylcellulose serving as a macromolecular emulsifying agent is reduced, a surfactant is not needed, and the prepared alkenyl succinic anhydride sizing agent has good emulsion stability and high sizing efficiency.