Abstract: Environmental pollution caused by petroleum-based film materials has increasingly attracted public attention in recent years. Therefore, the development of new, alternative fully biobased film materials is urgently needed and has gradually become a research hotspot in scientific fields. Using chitosan (CS) and hydroxypropyl methylcellulose (HPMC) as raw materials, anthocyanin (Anth) as an indicator, five of biobased composite film materials were prepared: Anthx-HPMC/CS (x = 0, 2, 5, 8, 10). The effects of Anth content on the structure, thermal stability, mechanical properties, water sensitivity, and pH sensitivity of pH-responsive films were systematically studied through rheological analysis, FTIR, XRD, water vapor transmission rate (WVP) testing, thermogravimetric analysis, mechanical property testing, and pH responsiveness testing. Furthermore, the pH-responsive films were applied for real-time monitoring of the freshness of shrimps. The results showed that Anth could interact with CS and HPMC at the molecular level, significantly enhancing the thermal stability and tensile strength of the biobased films, although the WVP values increased, leading to higher water permeability. When the Anth content reached 10%, the Anth₁₀-HPMC/CS film exhibited excellent pH responsiveness. Application of the Anth₁₀-HPMC/CS film in shrimp freshness detection revealed that as the shrimps transitioned from fresh to spoiled, the film color changed significantly from purple to yellow-green. This study holds theoretical guidance value and practical application significance for biosensor detection of food freshness.