The invention discloses a silver nanoparticle film and a preparation method thereof. The film takes a substrate as a main body, a silver layer grows on the substrate, silver nanoparticles are separated out by utilizing a reducing agent, and the silver layer is formed; and the surface, on which the silver layer is separated out, of the film serves as an inner packaging layer of marine products, andfreshness of the marine products can be monitored. The preparation method comprises the following steps: soaking the substrate in an n-butylamine solution, growing a silver nanoparticle layer on thesubstrate by utilizing a silver-ammonia method, reducing and separating out the silver nanoparticles, and forming the silver layer, thereby obtaining the silver nanoparticle film. According to the method disclosed by the invention, hydrogen sulfide gas released during seafood deterioration reacts with the silver nanoparticle film to produce the black silver sulfide precipitate, so that the brightsilver layer is blackened to monitor the freshness of the marine products. Compared with the traditional monitoring method, the film disclosed by the invention is used for monitoring the marine products, no specific experimental temperature or professional equipment is needed, and the film can be linked with the conventional marine product packaging, can be manufactured on a large scale and is high in cost performance.