To reveal the mechanism by which oblique inflow at bends affects the hydraulic characteristics of a single-orifice sluice, a three-dimensional hydrodynamic numerical model is constructed using the Erhe Hub Sluice on the Huaihe River Seaward Waterway as a case study. The flow field structure and water level variation patterns upstream of the sluice are simulated and analyzed under inflow angles of 30°, 45°, 60°, and 75°. A formula for calculating the comprehensive discharge coefficient of the sluice under oblique inflow conditions is established, considering factors such as inflow angle, Froude number, and width-to-depth ratio. The results indicate that the longitudinal velocity distribution pattern of lower velocities on the concave bank and higher velocities on the convex bank intensifies with increasing inflow angle. When the inflow angle ranges from 30° to 75°, the ratio of the velocity difference between the concave and convex banks to the mean cross-sectional velocity at the bend outlet increases from 16.5% to a maximum of 84.3%. The variation of cross-sectional circulation intensity with inflow angle differs under different discharge conditions. The water level at the bend outlet cross-section exhibits a lateral distribution with higher levels on the concave bank and lower levels on the convex bank. When the inflow angle ranges from 30° to 75°, the ratio of the water level difference between the concave and convex banks to the mean cross-sectional water level increases from 1.3% to a maximum of 30.7%, and the inhibitory effect of flow inertia on circulation decreases with increasing inflow angle. Under identical discharge conditions, the comprehensive discharge coefficient of the sluice decreases with increasing inflow angle. The average relative error between the actual values and the values calculated from the established formula for the comprehensive discharge coefficient of the sluice is 2.01%, demonstrating generally high accuracy.