The soil mixing wall (SMW) retaining structure is applied in two long strip excavations in Shanghai. Firstly, the bearing and deforming mechanism of SMW is analyzed in brief. The structural analysis method ofSMWis discussed. Secondly, based on the in-situ excavating construction procedures, the construction steps of excavating and supporting are simulated in the numerical calculation with the method of Fast Lagrangian Analysis of Continua 3D. There are two cases simulated in numerical calculation, case 1 is the normal case in which the supports are installed timely, and case 2 is a case in which the supports are not installed timely because of some reasons. Then, the deformation of the retaining structure, the horizontal displacement at the top ofSMW and the axial forces of steel pipe supports are analyzed and compared with the actual observation data in two cases. A good agreement can be found between the calculation results and observation data. It can be seen that in case 1 the excavation is stable and safe; the axial forces are lower than the alarm values and the displacement due to excavating is in the permissible range. In case 2, however, the excavation is in danger of instability and some measures should be taken to protect the SMW retaining structure from failure.