Abstract: The characteristics and modes of failure of deep foundation pits are introduced. The internal factors of accident reason are discussed. For the accident of subway foundation ditch, the existing problems in the design, construction and monitoring are analyzed. The main reasons of the accident are that the shear-resistance rungs are not installed according to the design requirements between steel wailings and soldier piles, resulting in that the joints between steel wailings and soldier piles cannot support effectively the tangential component force of axial force of diagonal bracings, and the sliding friction between steel wailings and side wall of ditch is aroused, so the structural supports lose bearing capacity. The diagonal bracings and corbels are not welded according to the design requirements. The mechanism of load-carrying of support system in the direction of gravity changes essentially, as a result, without effective anti-sliding constraints, the steel wailings exhibit overturning and unstability, and one end of the steel diagonal bracings falls freely. Deep foundation pits should enhance the process management and control of construction quality, related parties should enhance intercommunication, and construction units should grasp the main design intention and the key design link. The unusual underlying reasons of monitoring data should be comprehensively analyzed, and specific emergency measures should be proposed.Key words: deep foundation pit; damage of foundation pit; engineering accident; monitoring