This study applies polyacrylamide (PAM) solution to improve shield tunnel sandy soil. A multiscale analysis method, including macroscopic experiments, microscopic characterization, and molecular dynamics (MD) simulations, was employed to investigate the interfacial interaction between PAM molecules and shield soil particles, as well as the improvement mechanism. On the macroscopic scale, slump, permeability, and direct shear tests reveal that the optimal improvement conditions for the PAM solution are a concentration of 0.5 % and an injection ratio of 25 %. On the microscopic scale, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy techniques were used to uncover the microscopic mechanisms before and after the PAM treatment. The results show that the polar groups in the PAM molecules interact with the hydroxyl groups on the surface of the sandy soil particles, forming a flexible film resembling a cementing agent on the surface, filling the pores and improving the transition zone between particles. On the nanoscale, based on MD simulations, a (PAM, H₂O)/α-SiO₂ molecular interface model was established to quantify the interfacial interaction between PAM molecules and sandy soil particles. The findings show that the polar groups in the PAM molecules enhance the interfacial bonding force with the α-SiO₂ surface through hydrogen bonds. PAM molecules form a stable adsorption layer on the surface of sandy soil, limiting their movement and enhancing the interface stability, revealing the molecular-level mechanism of PAM solution in improving sandy soil.