As regional soils collapse during the development of Benggang landforms, the release of geostress results in soil disintegration. This phenomenon is associated with soil instability, and it necessitates a study on the mechanical behaviour of decomposed granitic soils in response to unloading in the Benggang area. This study addressed the short‐ and long‐term mechanical behaviour of granitic soils with various degrees of weathering in response to unloading. Triaxial unloading and multi‐stage unloading creep tests were performed, including a comparative study of consolidated undrained (CU) triaxial tests. The deformation characteristics of granitic soils differed with the degree of weathering and the initial stress level. The most significant adverse effect of creep behaviour under unloading and undrained conditions was found in granitic soils with the greatest loss rate of maximum shear stress in contrast to the cases of CU compression and one‐time unloading. Additionally, increasing time‐dependent deformation was observed with increasing degree of weathering and initial stress level, which suggested that creep behaviour under unloading conditions accelerated soil erosion and instability in the Benggang area. The results of this study highlighted the potential influences of geostress release and time‐dependent deformation on soil instability in the Benggang area and advanced the understanding of mechanical mechanisms during the development of Benggang landforms.