Sodium carboxymethyl cellulose (CMC) is cross-linked with pea protein isolate (PPI) to prepare pea protein isolate/sodium carboxymethyl cellulose (PPI/CMC) composite film by the diffusion method. The effects of the mass fraction of CMC on the mechanical properties and water resistance of PPI/CMC composite film are investigated. In the film containing 0.4% CMC, the water vapor permeability (1.43 × 10⁻⁷ g m (m²∙h∙Pa)⁻¹) is significantly decreased, while the water contact angle value is elevated (55.76°). The tensile strength of the composite film is enhanced by 54.45%, while the elongation at break is increased by 23.30%. The film has excellent thermal stability and a homogenous structure. The remarkable changes may be attributed to new chemical interactions (hydrogen bonds) between CMC and PPI as revealed by Fourier transform infrared spectroscopy. Further, the composite film has great advantages in water resistance after comparison. The film is applied to the fresh bean skin, effectively blocking and reducing the tearing force between the fresh bean skin, and consequently easy separation after soaking in water for 20 min. Overall, the inclusion of CMC effectively solves the functional limitation caused by the hydrophilicity of PPI film, paving the way for its effective application as a novel edible packaging film in conventional foods.