Ground loss induced by shield tunneling inevitably causes uneven settlement of existing structures. Significant challenges are posed to shield proximity construction by soft soil, which is characterized by its high compressibility, high sensitivity, and low strength. To address this issue, leveraging the project of Ningbo Metro Line 7 passing under the parallel subgrade section of the existing North Ring Downstream Line and the Hongtang Township Yard Pull-out Line. The deformation characteristics of the subgrade during shield tunneling, as well as the subgrade deformation before and after the implementation of reinforcement measures, were investigated through a three-dimensional numerical model considering the reinforcement effect of cement mixing piles and through field monitoring data. The results indicate that without any reinforcement measures, the settlement of the existing subgrade does not meet the requirements specified in the relevant codes. When the metro jet system (MJS) pile combined with compaction grouting method is adopted, the maximum cumulative settlement is reduced by 57% compared to the unreinforced scenario. The effectiveness of the reinforcement measures is also verified by the field monitoring results. The MJS pile combined with compaction grouting method adopted in this project can serve as a reference for shield proximity construction under similar complex conditions.