Abstract: The effective stress parameter χ quantifies the efficiency with which suction was converted into effective stress； clarifying its variation pattern and reliable test methods remains one of the core challenges in unsaturated soil mechanics. Because χ was extremely difficult to measure directly， experimental data were scarce. Using a complete set of measured shear strength and suction data obtained for a compacted yellow-brown expansive soil from Jingmen， the dependencies were established among χ， net stress， suction， void ratio e， and gravimetric water content w. The results showed that： ① Moisture effect： At a given density， χ increased with w， indicating that the efficiency of converting suction into effective stress rises with increasing water content. ② Density effect： At a fixed moisture level， χ rised markedly as e decreased， demonstrating strong density dependence-higher density significantly enhances the suction-to-stress conversion efficiency. ③ Stress effect： Under constant e and w， χ increased with vertical net stress， implying that the external stress state also influences χ； the mechanism was analogous to the density effect because greater vertical stress induces specimen compaction. ④ Peak （“mountain”） effect： Unsaturated shear strength exhibited a “peak phenomenon” because χ increased whereas matric suction decreased with rising moisture. At low moisture， χ was small and suction was large； at high moisture， χ was large and suction was small. Consequently， suction-induced strength reached a maximum over the full transition from saturation to air-drying， producing a corresponding peak in unsaturated shear strength. ⑤ Research outlook： Because χ was closely linked to density， moisture and stress state， developing more rational quantitative descriptions of these interrelations warranted further investigation.