Heat-induced polymerized whey protein (PWP) gels from whey protein isolate have limited application due to their high brittleness, but addition of polysaccharides in PWP gels can significantly improve their properties. Agaricus bisporus polysaccharide (ABP), as an anionic polysaccharide, exhibits excellent biological activities, but the formation mechanism of its composite hydrogels with PWP remains unclear. In this study, the effect of ABP concentration (0-4 g/100 mL) was investigated on physicochemical properties of PWP-ABP composite hydrogels including average particle size, zeta potential, surface hydrophobicity, intrinsic fluorescence spectrum and free sulfhydryl group content. The results demonstrated that as the ABP concentration increased, the average particle size of PWP-ABP composite hydrogels significantly increased from (76.22 ± 7.43) to (145.93 ± 8.20) nm (P < 0.05), and the absolute value of zeta potential rose from (35.60 ± 2.64) to (45.20 ± 1.40) mV, indicating that ABP enhanced the stability of the composite hydrogels through electrostatic repulsion. Additionally, the surface hydrophobicity decreased remarkably, and the free sulfhydryl group content significantly decreased (P < 0.05), confirming that ABP altered the tertiary structure of PWP via hydrophobic interactions and disulfide bond crosslinking. Synchronous rheology-Fourier transform infrared spectroscopy analysis revealed that ABP induced a red shift in the amide A region (3600-3200 cm^-1) of PWP, suggesting enhanced hydrogen bond formation; the fluctuations in the amide I band (1625 cm^-1) were attributed to electrostatic interactions. Molecular docking analysis showed that ABP binds to β-lactoglobulin via hydrogen bonds and two-dimensional correlation spectroscopy further validated the changes in O-H stretching vibrations. In conclusion, ABP optimizes the gel network structure of PWP through hydrophobic interactions, hydrogen bonds and electrostatic interactions, providing a theoretical foundation for developing functional food gel systems.