Abstract: During new metro line construction, unavoidable disturbances are generated in existing line surrounding stratum, causing certain level of deformation to existing line and subsequently causing a series of disease problems. To ensure existing line safe operation, it is necessary to study the law of existing line displacement and deformation induced by metro shield tunnel overpassing construction. Method: Taking Changsha Rail Transit Line 6 shield tunnel overpassing Line 3 in small clearance as example, a threedimensional numerical model is established using FLAC3D software. In the above overpassing process, the existing line vertical deformation development law and the reinforcement effect of circular reinforcement control measures are analyzed. Result & Conclusion: The main affected area of the existing metro tunnel is in the vicinity of the horizontal projection intersection point between the existing and new tunnels. The impact on existing tunnel is significant when the construction distance between the new and existing shield tunnels is within 12.0 meters on each side. Therefore, during construction in this range, it is necessary to focus on controlling the uplift deformation of existing tunnel. It is recommended to use 5 reinforcement rings. The circular reinforcement structure does not change the distribution law of the existing tunnel vertical displacement but can reduce the maximum vertical deformation of the segment to 1.1 mm. After reinforcement, the maximum vertical displacement at existing line arch crown is only 1.3 mm, and the maximum horizontal displacement at the arch waist is only 1.4 mm, with small displacement values observed at all monitoring sections. The adopted construction control measures effectively reduce the construction disturbance caused by the new tunnel overpassing existing line engineering, ensuring the construction safety.