Abstract: The running tunnel between Qingdao North Station and Cankou Station of the new Qingdao Metro Line 8 passes under the existing operational Metro Line 3 in close proximity, with the distance between the initial support of new metro tunnel and the bottom floor structure of Line 3 being only 0.4 m, which is extremely challenging for construction without affecting the normal operation of the existing Metro Line 3. By constructing a 3D finite element model, this paper simulates the full construction process of the new running tunnel passing under the operational existing line. And relevant targeted measures are prepared: using non-blasting mechanical excavation to reduce vibration; strengthening advance support and fast-closing support ring to control settlement; and automatic monitoring structural deformation of Line 3 to provide rapid feedback and guide on-site construction. Through on-site technical and safety control means, the maximum settlement of the tunnel structure of Line 3 during the underpass construction was only 2.11 mm, and the maximum settlement of the track bed was only 1.55 mm, which satisfied the requirements for settlement control without affecting the safe operation of Metro Line 3 during the construction period. The relevant engineering experience can provide a reference for design and construction of similar tunnel works passing under existing operational metro tunnels with small clearance, small angle and long distance.