<p>Under cold-region engineering and artificial ground freezing conditions, frozen sandy clay is frequently subjected to complex three-dimensional nonhydrostatic stress states, and its macroscopic mechanical behavior is intrinsically linked to microstructural evolution. In this study, true triaxial tests were conducted on frozen sandy clay under varying confining pressures and intermediate principal stress coefficients (<i>b</i>) to clarify their regulatory effects on strength, deformation, and microstructure. Complementary analyses, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR), were performed. The results indicate that frozen sandy clay exhibits continuous strain-hardening behavior. The peak deviatoric stress increases initially and then decreases with increasing <i>b</i>, reaching a maximum at <i>b</i> ≈ 0.5. Higher confining pressure significantly enhances strength and mitigates post-peak softening. Microscopically, increasing <i>b</i> promotes crack closure and structural densification, thereby reducing porosity. In contrast, higher confining pressure induces particle crushing and micropore refinement, leading to an increased pore fractal dimension. NMR results show that increasing b shifts the main T₂ peak leftward and reduces the proportion of medium and large pores. Increasing confining pressure causes a rightward shift of the T₂ cutoff and expansion of the adsorbed water interval, suggesting enhanced water film thickness on particle surfaces. These findings provide a theoretical basis for the design and stability evaluation of deep artificial ground freezing projects.</p>