A synthesized investigation is conducted of trench formation machine’s effect on trench wall in soft soil area. The failure pattern of the slurry-trench is analyzed by means of 3D FEM based on strength reduction method. The investigation shows that the global instibility occurs in the superficial layers and speads along the trench segment. The grab movement in slurry has an instant impact on the trench wall and may cause local instability. The overload and construction technique of trench formation equipment are both important concerns during trench construction to ensure safetry.