Owing to the continuous consumption of fossil fuel and increasing environmental problems, there is an urgent need to develop technologies for the efficient use of clean energy. In this study, a mono-tangent backward-rotating impeller based on tidal current energy utilization is proposed, and its hydrodynamic performance was studied using the computational fluid dynamics method by establishing a hydrodynamic model of the backward-rotating impeller. The influences of the number of blades, impeller aspect ratio (λ), and different flow speeds on the coefficient of power (cp) of the mono-tangent impeller were investigated. The results show that the value of cp of the mono-tangent impeller with four blades is higher than that of the impellers with two, three, five, and six blades. λ has a significant impact on the backward-rotating impeller, with cp of the impeller reaching an optimal value when λ = 0.5 and the flow speed is 1.25–1.5 m/s (with a tip speed ratio of 0.6–0.75).