In mountainous western China, a large number of granular materials are used as construction materials in high-fill embankments. These granular fill materials in the embankments are typically unsaturated owing to the arid and semi-arid climates in this area. However, previous studies on unsaturated soils primarily focus on fine-grained soils. In this study, a series of triaxial tests were performed to determine the critical state parameters of a granular fill material in q–\( \bar{p} \), v–ln \( \bar{p} \), and v_w–ln \( \bar{p} \) planes. An upgraded double-cell triaxial system (DCTS) was used in net confining pressures ranging from 0 to 450 kPa and matric suctions ranging from 0 to 160 kPa. This study demonstrates the good performance of the upgraded DCTS in unsaturated soil testing. Experimental results show that the critical state lines are almost parallel to those of saturated soil in the q–\( \bar{p} \) plane, suggesting that the friction angle is independent of suction. The total cohesion and hence the shear strength increase with suction. In the v–ln \( \bar{p} \) plane, both the intercept and slope of the critical state line increase with suction. Finally, it is observed that the intercept and slope decrease with increasing suction in the v_w–ln \( \bar{p} \) plane.