An improved method for iterative calculation of the m value is proposed based on the conventional flexibility fulcrum method. In a case study of a deep foundation pit at a subway station in Nanjing City, the results of the two calculation methods were compared with observed data. The results show that the improved flexibility fulcrum method can simulate the changes of the interaction between soil and the supporting structure during the deformation of the supporting structure to a certain extent, which is closer to the reality. The iterative method can better predict the deformation of the foundation pit, which is helpful to the computation theory based on the controlled deformation.