<p>The Loess Plateau region faces persistent geohazards that critically threaten both ecological stability and human safety, largely due to the distinct metastable microstructure of loess, which evolves under changing environmental conditions and governs its macro-mechanical behavior. Unlike traditional models that rely solely on macroscopic parameters, this study introduces the ratio of large and medium pore volume (<i>PLM</i>) as a key predictor alongside moisture content (<i>w</i>) to establish a novel empirical model for loess shear strength. Laboratory tests were conducted on undisturbed loess specimens with controlled moisture content and pore structure characteristics. The results demonstrate that increased moisture content and a higher proportion of large and medium pores significantly reduce shear strength, revealing the critical role of pore space morphology in mechanical degradation. A new method for accurately predicting the shear strength of unsaturated loess using easily measurable parameters has been developed based on mechanical analysis and experimental data. This method provides a practical, microstructure-based tool for evaluating the stability of surface slopes and foundations in loess areas, and has wide applicability in geotechnical engineering.</p>