Objective To investigate the effects of carboxymethyl pachymaran on the stability, structure, physicochemical properties and biological activity of casein at different ratios. Methods Pachymaran was derivatized with carboxymethyl groups to enhance its solubility, and its structural features were studied; carboxymethyl pachymaran was mixed with casein in different ratios, and the turbidity, zeta potential, protein solubility, and phase diagram of the complex were evaluated at different pH values; the aggregation degree and physicochemical properties of different casein-based samples were studied under acidic conditions using confocal laser microscopy and rotational rheometer; free radical scavenging ability and non-enzymatic glycation product inhibition ability of different casein-based samples were compared. Results The molecular weight of carboxymethyl pachymaran obtained in this study was 2.43×10⁵ g/mol. The successful preparation of carboxymethyl pachymaran was determined by the degree of substitution (0.84) and the characteristic absorption peak of carboxymethyl group. When carboxymethyl pachymaran was mixed with casein, it could effectively affect the pH stability and reduce the turbidity of casein under acidic conditions, and its solubility was also enhanced. This might be due to the electrostatic interactions between polysaccharides and proteins affecting the surface charge distribution of the molecules. Among them, the best stabilizing effect was observed when carboxymethyl pachymaran was mixed with casein at 5:1 (w/w). Subsequent studies on the structure and physicochemical properties of the complex showed that with the introduction of polysaccharides, especially when they were mixed at a ratio of 5:1 (w/w), the aggregation degree, apparent viscosity and modulus of the complex were lower, indicating that its gel network structure had better stability. The biological activity results showed that the complex, especially when they were mixed at a ratio of 5:1 (w/w), exhibited the strongest antioxidant activity and non-enzymatic glycation product inhibition ability. Conclusion The carboxymethyl pachymaran-casein complex obtained in this study has better structural stability and biological activity in acidic environments, providing a theoretical basis and technical support for the development and processing of stable acidic dairy products.