In order to investigate the influence of TBM pilot tunnel excavation and enlargement construction on adjacent structures in a site containing existing buildings, this study takes the Buji south ring road tunnel project as a case study. A three-dimensional numerical model incorporating upper buildings and an underlying water supply tunnel system was established using finite element software. The model simulated and analyzed ground settlement, tunnel convergence, surrounding rock stress characteristics, and the impact on both upper buildings and the underlying water supply tunnel during various construction stages of TBM pilot tunneling and enlargement. Monitoring data were used to validate the numerical simulation results. The findings reveal that: 1) During TBM pilot tunneling and enlargement construction, ground settlement exhibits a superposition effect and increases nonlinearly with construction steps. The settlement induced by tunnel enlargement is significantly greater than that caused by TBM pilot tunneling, and the settlement generated by the left-tunnel excavation exceeds that of the right tunnel. The convergence values of each pilot tunnel vary upon completion of the four-tunnel excavation. Notable stress concentration occurs at the tunnel haunches, arch feet, and invert. 2) TBM pilot tunneling and enlargement construction exert a measurable impact on both upper buildings and the underlying water supply tunnel. Building settlement shows a clear increasing trend with construction progress, reaching a maximum settlement of 4.53 mm after tunnel enlargement. The vertical displacement of the underlying water supply tunnel exhibits a gradient distribution along the pilot tunnel axis, decreasing from the crown to the invert. The enlargement phase induces significant uplift in the water supply tunnel, with a maximum uplift of 1.11 mm post-enlargement. The research findings of this paper can be applied to practical tunnel excavation and support processes to ensure the safety of the tunnel and surrounding structures during construction.