The invention belongs to the technical field of chemical engineering, and relates to a preparation method for denitration auxiliary agent for removing NOx in catalytically-cracked flue gas. The preparation method comprises the following steps: preparing a large-dimension Na type pure silicon ZSM-5 molecular sieve; preparing HZSM-5 pure silicon molecular sieve through ion exchange; preparing a copper-lanthanum-cerium-aluminium-iron or nickel-lanthanum-cerium-aluminium-iron dipping solution; and preparing the denitration auxiliary agent, namely, dipping the HZSM-5 pure silicon molecular sieve in the dipping solution, and completely removing water in the molecular sieve after dipping is finished, performing vacuum dewatering or baking, and then sintering the molecular sieve, so as to obtain the denitration auxiliary agent for catalytically-cracked flue gas. Active compositions are high in dispersity, metal oxide active compositions are uniformly distributed in the molecular sieve channels, and the catalytic efficiency is high. The denitration active compositions are dispersed in the channels of the pure silicon HZSM-5 molecular sieve, heavy oil molecules cannot enter the channels, and the denitration active compositions cannot cause adverse influence on distribution of the catalytic cracking product, and can be added according to the actual demand on the denitration auxiliary agent.