Abstract: The stress distribution and variation inside loosely deposited construction waste soil are highly complex. In particular, the continuous filling process alters the initial shear stress, which may significantly change the instability behavior of the waste soil and pose serious challenges to its overall stability. To investigate the influence of initial shear stress on the undrained instability characteristics of construction waste soil, a series of consolidated undrained triaxial tests are conducted on five types of construction waste soil with different particle gradations under four initial shear stress condition. The results show that poorly graded waste soil is more susceptible to undrained shear instability, exhibiting lower strength and smaller strain at instability state. As the initial shear stress increases, the critical state line shifts toward the higher strength region. Meanwhile, the stress ratio at the onset of instability under undrained conditions increases, whereas the corresponding axial strain and pore water pressure decrease. Considering the effects of initial shear stress, undrained shear strength, and shear stress increment, an undrained stability index for construction waste soil is proposed. This index provides a rational approach for evaluating the undrained stability of slopes made of construction waste soil under varying stress conditions.