Abstract: Stability of the slurry trench in complex stratum determines the quality of the diaphragm wall during the construction progress. It is of great significant to study the influence of the unit weight of slurry and soil characters to the local stability of slurry trench. Base on the instability mechanics of the slurry trench, damage-instability model for the trench is presented using the maximum limit analysis method. Both local factor and critical height of the slurry trench are proposed, which combines the nonlinear M-C failure criterion and strength reduction method. This approach is compared to other two methods with applying them to a project practice. The relationships between the local factor, critical height and the unit weight of the slurry, and character parameters of the soil are obtained. Results calculated by all the methods show that, safety factor increases as the increment of unit weight of slurry, cohesion and compressive stress of the soft soil, and decreases as the increment of thickness and unit weight of the soft intercalated layer. However, the calculated results by this approach is sensitive to the change of unit weight of the slurry, which is closed to the phenomenon in site. The local factor is generally the minimum to the others. Furthermore, Comparison of this approach with the method on the basis of the M-C criterion indicates that the nonlinear character of the soft soil only affects the size of local failure area, not to its basic shape.