Abstract: Massive quantities of waste soil generated during the construction of the Pinglu Canal project are temporarily or permanently stockpiled in nearby disposal sites. Under fluctuating hydraulic conditions, these loosely deposited soils are highly susceptible to instability, posing significant challenges to the safety of slopes. Due to various factors such as construction practices and deposition processes, the stress distribution within slopes is inherently complex. In particular, the progressive accumulation of material induces evolving initial shear stresses that may critically influence the soil’s instability characteristics. To investigate the effect of initial shear stress on the undrained instability behavior of engineering waste soil, a series of consolidated undrained triaxial shear tests were conducted on five types of spoil with varying particle gradations under four levels of initial shear stress. The results indicate that poorly graded spoil is more prone to undrained instability, exhibiting lower shear strength and smaller axial strain at instability. The presence of initial shear stress shifts the critical state line toward higher strength regions in the q–p' space, suggesting a strengthening effect. As the initial shear stress increases, the stress ratio at instability rises linearly, while the corresponding axial strain and pore pressure decrease. Based on the combined effects of initial shear stress, undrained shear strength, and disturbance stress, a stability index under undrained conditions is proposed. This index provides a rational framework for evaluating the stability of spoil deposits under undrained loading.