The invention relates to a method for modifying a polyamide composite thin film, relates to a method for improving pollution resistance of a forward osmosis polyamide composite film through post-treatment modification, and is used for solving the problems that because a conventional polyamide TFC film easily gets biological pollution, the film effluent flux is decreased, the film cleaning frequency is increased, the energy consumption is increased, the film life cycle is shortened, then the process treatment efficiency is reduced and the operation cost is increased. The method comprises the steps: one, coupling dopamine and an initiator; two, introducing the initiator to the surface of a film; three, grafting a zwitterionic polymer on the surface of the film; four, adsorbing silver ions; and five, producing nano silver in situ. The obtained modified TFC film has anti-adhesion and bacteriostasis effects, and can effectively control the biological pollution of the film. The atom transfer radical polymerization reaction is adopted for grafting of the zwitterionic polymer, large controllability is achieved, the graft length and density of the organic polymer can be controlled by adjusting and controlling parameters, and the properties of pollution resistance and mass transfer of the film are further optimized.