The exploration and design of tunnel in the submarine karst area are difficult and the construction risk is high. Hence, the crosshole CT geophysical prospecting + drilling method is adopted for the first time in largediameter submarine karst shield tunnel on Dalian Metro Line 5. And then the reinforcement range of the tunnel is determined by finite element method; the reinforcement standard of karst tunnel foundation is determined by antifloating calculation; and a specific implementation scheme of submarine karst pretreatment and the specific design scheme of shield construction are put forward. In this way, a whole set of exploration, design and treatment scheme of submarine karst shield tunnel is formed.