Aimed at the lack of numerical calculation of deep foundation pit stability and anchor rod stress in Yinchuan area, the excavation-dewatering process of a deep foundation pit of an underground garage in Yinchuan, Ningxia Autonomous Region, which is reinforced by anchors and nails, was numerically analyzed using the HS-Small constitutive model of the Midas GTS NX software. The results showed that the deformation of the foundation pit slope increased gradually with the excavation, and the maximum displacement appeared at the top external corner of the foundation pit. The axial force of the anchor rods increased with the increase of excavation depth, and the ability of controlling deformation was related to the positioning of the anchors. The ability of controlling deformation of the lower part of the anchor rods was better than that of the upper part. There was a large difference in deformation between the support of soil nails and the combined support of anchors and nails. The settlement of surrounding buildings increased slowly and the final settlement was less than the warning value with the excavation and continuous dewatering. The axial force of soil nails in the combined support showed a slight S-shaped curve, and the maximum value was located at about 3/4 of the soil nail length. According to the comparion analysis of the simulation results and the measured data, the results are in good agreement. The parameters of HS-Small model of typical soil layer in Yinchuan area obtained by back analysis can be used for the numerical simulation of similar deep foundation pit engineering in Yinchuan area.