Excavation often leads to severe disturbance of the existing tunnels while undercrossing them at zero distance. As the utilization rate of underground space increases, it is inevitable that there will be close fitting construction between new and old structures. By designing reasonable construction plans, standardized construction, and detailed monitoring, the deformation of existing structures can be effectively controlled. Based on the construction of the new platform layer of Guangzhou Metro Line 12 at Chigang station, which underpasses through the existing subway tunnel, a reasonable construction method was designed, and the entire process of structural monitoring was carried out during the construction. The results show that the numerical simulation method predicts that the vertical displacement of the existing tunnel and the axial force of the support column are within the safe range. The on-site monitoring results show that the maximum vertical displacement of the tunnel reaches about 6 mm, and the horizontal displacement remains stable. The settlement difference of the track increases during the excavation stage of the underpass, with a maximum settlement difference of about 4 mm. The cross-section of the left and right tunnel sections shrinks by 4 mm and expands by 2 mm respectively, with a vertical convergence of 4.5 mm.