Stress in the media is one factor which limits the performance of high-power solid-state lasers. The piezooptic tensor of reduced-suffix matrix form in laboratory coordinate system was revised, and the depolarization loss induced by assembly-stress was analyzed under plane-strain approximation. The results indicate that great stress will induce unignored depolarization loss. Especially the depolarization induced by shear stress cannot be reduced by controlling the angle between the crystalline [101] axis and the x axis of the laboratory coordinate. When assembling the slab and heat-sink, it is necessary to reduce the assembly-stress, especially the shear stress.