Weight reduction is important when additive manufacturing becomes feasible for marine turbine runner. Hydrofoils are usually used as the simplified model of turbine blade for investigating complex problems. A NACA0015 hydrofoil is used to explore the weight reduction by setting internal structural support instead of the solid foil. Firstly, computational fluid dynamics (CFD) simulation is conducted to have the hydraulic loading. Then, fluid-solid interaction (FSI) method is used to evaluate the stress and deformation of the initial solid foil. Rib plates are used as the internal structural support with two main parameters that the rib width trib and angle of rib γrib. The global dynamic-criterion (GDC) algorithm is used for optimizing the uniformed maximum stress SND and the uniformed weight WND of foil. Finally, the optimal foil is found with trib of 1.716 mm and γrib of 69.44°. The value of SND is 1.595 times of the initial foil and WND is reduced by 46.6%. A compromise solution is effectively found for both sufficient strength and lower weight. Grey relation analysis shows that the influence of trib is stronger than that of γrib especially for the weight. This study helps the additive manufacturing of marine turbine runner and blades.