Haicang tunnel, which is to be built in Xiameng, China, is the second subsea tunnel in Xiameng. The tunnel passes through complex geological conditions under the sea and passes underneath sensitive buildings. In the paper, the possible maximum longitudinal gradients of Haicang tunnel are studied so as to achieve the optimum maximum longitudinal gradient. Similar project cases at home and broad are surveyed, engineering analogy and theoretical analysis are made, the construction risks, construction schedules, construction cost, operation cost and vehicle operation cost of the tunnel under different maximum longitudinal gradients are studied by means of full life cycle design concept, with relevant specifications and standards at home and abroad, the passing capacity of the tunnel, the safety of the vehicle operation and the minimum cover depth considered. It is concluded that 3.5% maximum longitudinal gradient, which can result in low construction risks and optimum economic efficiency, should be adopted for Haicang tunnel. The study result can not only provide support for the selection of the maximum longitudinal gradient of Haicang tunnel, but also can provide reference for the design of similar subsea tunnels in the future.