Abstract: Objective: Metro longlarge interval undersea tunnel features long seacrossing distance complex geological conditions and marine environment. The tunnel design is difficult, construction has high risks, and is in great need of referable engineering cases or construction experiences. Thus it is necessary to carry out indepth research on key design technologies for the metro longlarge undersea interval such as civil engineering, ventilation, disaster prevention, water supply and drainage, and fire protection. Method: Based on the engineering overview, geological conditions, marine environment, and design standards of Qingdao Metro Line 8 (hereinafter referred to as ′Line 8′) undersea interval tunnel, four construction methods are compared and selected, including shield tunneling method, mining method, immersed tube method, and cofferdam openexcavation method. Upon determining the construction method, responsive measures are further formulated according to the complex geological conditions of the seabed, including the construction crossing large undersea fault fracture zones, longdistance excavation in highpressure and complex stratum using slurry shield tunneling, and the construction of undersea section link passage in soft stratum. Key design technologies in ventilation, disaster prevention, water supply and drainage, and fire protection are proposed. Design concepts are innovatively proposed, including the use of mechanical pipe jacking method to build undersea link passage, and cutter replacement under pressure by undersea high waterpressure saturated gas. Result & Conclusion: It is difficult to deal with the complex and everchanging geological conditions of metro longlarge undersea tunnels by one construction method. It is recommended to adopt different construction methods according to their responsive capabilities to the undersea geological conditions and build undersea tunnel with combined construction plan.