The measured steel strut stress is often less than the design value of steel strut prestress in deep excavation engineering. The insufficient of steel strut prestress has an important effect of the steel strut reducing the large deformation of the retaining wall of excavation. A two-dimensional numerical calculation model as a subway station case in Nanjing was established by using Midas GTS software. The suitability of the calculation model and parameters was verified by the comparison of the field monitoring data and numerical calculation results. Then the influence of the strut prestress value on the horizontal displacement of the retaining structure and the final steel strut stress was analyzed. The results show that the steel strut prestress is an effective method to overcome the small compressive stiffness of steel strut in the theory and improve the actual compressive stiffness of steel strut supporting. Taking the design value of steel strut prestress as a reference, the prestress ratio has a negative linear relationship with the maximum horizontal displacement of the wall, and insignificant influence on the final value of strut stress. The economy is the best for 40% of the prestress ratio in the effect of strut prestress. The low and high bidirectional warning method and the corresponding strut prestress ratio are also proposed on basis of the strut stress particularity.