To investigate the deformation effects of constructing a new type of connection passage in overlapping tunnels on nearby buildings, the first external connection passage in overlapping tunnels constructed in soft soil areas in China was studied. Key control techniques were summarized, including the “bottom-up” excavation sequence, simultaneous lower tunnel freezing and upper tunnel drilling, and cross-construction combining lower tunnel excavation with upper tunnel freezing. The environmental impacts of this novel connection passage were analyzed based on measurements. The results show that the displacement timeline of the ground surface along the central axis of the connection passage during construction is divided into seven phases: slight variation during upper tunnel drilling, frost heave during upper tunnel freezing, subsidence during upper tunnel drilling, fluctuating subsidence during excavation, frost heave during upper tunnel freezing, rapid subsidence during upper tunnel excavation, and stabilization during upper and lower tunnel grouting. Ground and adjacent structural deformation initially increases and then decreases, with the peak deformation occurring during the frost heave stage of the upper tunnel. Monitoring and control during this phase are recommended to avoid damage to nearby structures. The freezing construction of the external connection passage compresses the main tunnels, leading to “egg-shaped” vertical deformation in both the upper and lower main tunnels. During the freezing stage of the lower tunnel connection passage, the arch bottom of the lower tunnel sank while the arch crown rises. In the frost heave phase of the upper tunnel connection passage, subsidence is observed in the vertical displacement of the arch bottom and arch crown of the lower tunnel. The use of the “bottom-up” excavation sequence and associated control techniques reduces the construction period by 40%. The adverse environmental effects of constructing the upper and lower connection passages offset each other, reducing the disturbances to the surrounding soil layers and critical structures. This innovative technology for constructing connection passages in overlapping tunnels provides important reference value for future underground space development in dense urban areas.