In this study, stable sodium chlorite solution and water-borne polyurethane (WPU) were mixed to prepare a novel antibacterial film with sustained-release properties via a solvent casting method. For the first time, carboxyl in WPU was used as the activator to react with ClO− to produce gaseous ClO2 under the trigger of moisture and without any other acid reagent. The results showed that the film had excellent antibacterial effect. However, as the content of stable ClO− increased, the mechanical strength and thermal stability of the film decreased, and its surface morphology showed irregular protrusions. In addition, the slow-release mechanism of the film with time under different temperatures was studied, whereby the release amount of the film was noted to be affected by temperature. This study explored the development of food packaging with efficient antibacterial activity and sustained-release performance without the use of acid reagents.