Abstract: Under the design methods recommended by Shanghai standard code for design of excavation engineering, the ultimate state equations for the overall stability, basal heave stability and overturning stability of flexible retaining structure of excavations are establisbed. Considering the variability of variables (mainly cand φ), the reliability of the supporting structure of the excavations are calculated. By applying this method (FOSM method or JC method) to six excavation cases of Shanghai soft soils, the range of the stable reliability index of an excavation pit and the variation tendency of the reliability index are concluded. The target reliability index of the excavation cases in Shanghai soft soils are proposed. The results may provide a theoretical basis for the reliability evaluation of practical excavations and a basis for the determination of partial factor of the excavation.