The main pier cap of Zhuhai Honghe Bridge is located in the flow silt area of the Xijiang River Basin of the Pearl River, and the steel sheet pile cofferdam is used for the foundation pit support. During the excavation of the foundation pit, the steel sheet pile cofferdam was obviously deformed. After detailed analysis, it was found that the main causes for the accident were significant decrease in the mechanical properties of soil due to continuous rise of the groundwater level, insufficient length of the steel sheet pile in the side span, excessive concentrated load on the edge of the foundation pit, and insufficient construction accuracy of the support system. In order to ensure the safety of deep foundation pit support, the reinforcement measures for the steel sheet pile cofferdam were adopted in regard to the actual condition through comprehensive analysis of the causes for the deformation of the steel sheet pile cofferdam, including extension of the steel pipe pile cofferdam into the muddy soil laye, strengthening of the internal support system, which were continuous monitored during the construction process. Finally, the foundation pit construction was completed safely.