Based on a 3-D solid finite element parametric model, an optimization design method of reinforced concrete aqueduct with multi-longitudinal beams is proposed. In this method, the stress of sidewall and subplate is controlled by the sectional crack-resisting criteria, the space between longitudinal beams or crossbeams is decided by the coordination of structural deformation, and the bottom stress of longitudinal beams or crossbeams is restrained by the nominal tensile stress. Taking the lightest dead weight of aqueduct as the objective of optimization, this method is able to give the optimal sizes and distribution patterns of the main load bearing members. The internal force of each structural element of the optimized aqueduct is calculated by a 3-D beam-shell finite element numerical model and thus the reinforcements are arranged. Compared with the prototype, the optimized aqueduct shows the advantages of lighter weight, more reasonable stiffness distribution, coordinated deformation and economical reinforcements.