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As a byproduct of animal slaughter, bone has highquality protein content along with comprehensive nutrients. It will cause serious environmental issues and waste of resources if handled improperly. Collagen polypeptide (CP), prepared by enzymatic hydrolysis of animal bones, is respected in the fields of material applications, such as food, films, medicines, and composites, which is principally because such materials are considered to be promising solutions to environmental impacts of synthetic polymers. However, the molecular weight distributions of CP during the extractions of waste bone proteins may affect their physicochemical properties, mainly including solubility, viscosity, gelling ability and stability, thus significantly affecting the comprehensive performances of the composites. Therefore, it is of great significance to determine the optimal molecular weight of CP and explore its underlying mechanism of action in composites. The effects of molecular weight distributions of bovine bone CP on mechanical properties, water solubility, thermal stability, compatibility and microstructures of CPcarboxymethyl cellulose (CMC) composite films were investigated by using conventional methods, such as the Fourier transform infrared (FTIR) spectroscopy, Xray diffractometry (XRD), and contact angle determination. A series of experimental composite films with a range of different molecular weight distributions of CP was fabricated by the filmcasting method. The results showed that the addition of CP would significantly improve the mechanical properties and hydrophobicity of the composite films. The microstructures of the CP-CMC composite films were smooth and compact when the molecular weight distribution of CP was 700~1000u. Meanwhile, the tensile strength, breaking strength and Youngs modulus were about 2.39, 2.55 and 10.11 times higher than those of the control group, respectively, while the elongation at break was 40% lower than that of control group, also, the hydrophobicity and thermal stability performed the best. With the increase of molecular weight distributions of CP, the water solubility of the CP-CMC composite films was increased, and the mechanical properties and thermal stability were reduced significantly, which may limit its industrial applications. The research results would provide a useful theoretical foundation and basic data to support the efficient utilization of animal bones and promote the transformation of CP into material utilization.