The invention discloses a longsnout catfish fillet coating antistaling agent, a preparation method and a resh-keeping method thereof. Specifically, the longsnout catfish fillet coating antistaling agent comprises the following components in terms of the concentration in an aqueous solution, 10-25g/L chitosan, 1-3g/L a rosemary extract, 2.5-5g/L glycerol, 10ml/L acetic acid, 2.5-5g/L Tween-80, and water as the solvent. According to the invention, the rosemary extract is added into the chitosan solution to prepare the composite coating antistaling agent, and the bacteriostatic and antioxidant capacity of the coating antistaling agent can be greatly improved. After coating of longsnout catfish fillets, microbial growth can be inhibited, the grease oxidation rate can be slowed down, the juice loss is reduced, and longsnout catfish fillets can maintain good quality. By storing the longsnout catfish fillets using the composite coating antistaling agent under an environment of 0-5DEG C, the shelf life can reach 10-14d. The longsnout catfish fillet coating antistaling agent provided by the invention is simple to use, easy to operate and low in cost, and is conducive to commercial application.