The work aims to develop a visual freshness indicator film and explore its feasibility in the freshness monitoring of clams. With blueberry anthocyanins sensitive to pH as the indicator and chitosan as the substrate, five smart indicator films were prepared by incorporating gelatin, Nisin, and rosemary essential oil. Then, the film properties such as pH sensitivity, color responsiveness, microstructure, barrier properties, mechanical properties, water content and water solubility and antioxidant and antibacterial properties were investigated. The color of blueberry anthocyanin solution responded significantly to pH in the range of 3-12. The roughness of the microstructure of the film increased with the addition of components, while the water vapor barrier properties gradually decreased. The addition of Nisin and rosemary essential oil significantly increased the antioxidant and antibacterial abilities. The chitosan/Nisin/rosemary essential oil-blueberry anthocyanin (CSNR-ATH) film exhibited better UV barrier properties and lower water solubility. The CSNR-ATH film could reflect the freshness of clams sensitively during the refrigeration process, and the composite indicator film changed from light green to yellow-green. Therefore, the prepared smart indicator film (CSNR-ATH) provides a new choice for the freshness monitoring of clams.