In this paper,the support system for an over size deep excavation in Nanjing soft soil district is introduced in detail.Different soil parameters are used for calculation of typical cross-sections.The result of calculation shows that soil parameters have an enormous influence on the safety and economy of the support system and the soil parameters should be selected according to actual situation.At last,the accidents of deep excavation are described and relevant countermeasures are put forward based on analysis results,which can provide example for design of deep excavation in soft soil district.