In this study, a 2D fluid-solid-coupled numerical model was established based on the investigation data of a station of Tianjin metro line 3, and a series of numerical simulations were carried out to investigate the distribution of horizontal foundation modulus along the depth under conditions of different foundation pit width and dewatering depth. The numerical results are compared with those computed by the m method, which is a traditional design method of foundation pit recommended in the Chinese Code. The results indicate that under the common conditions of pre-excavation dewatering(PED), the horizontal foundation modulus decreases greatly compared with the values determined by the m method, and the larger the pit width and dewatering depth, the greater the decrease extent. When using the elastic foundation beam method to predict the PED-induced deformation of a retaining structure, the obtained deformation will be underestimated, if the m method is used to determine the horizontal foundation modulus.