To investigate the influence of the bottom slope ratio of the upstream and downstream connecting sections on the hydraulic characteristics of large interchange culverts, the interchange culvert of the Binhai Hub of the Huaihe River Sea-Entry Channel (Phase II Project) was taken as the research object. A parametric study on the bottom slope ratio of the upstream and downstream connecting sections was conducted using a 1∶30 normal-section physical model and the OpenFOAM three-dimensional numerical model. The hydraulic characteristics of the culvert under different flow rate and water level conditions were analyzed in terms of flow capacity, velocity distribution, sectional energy, head loss, turbulent kinetic energy, and turbulent dissipation rate. The results show that the total head loss coefficient of the culvert exhibits a nonlinear variation pattern, first decreasing slowly and then increasing rapidly, with the increase of the bottom slope ratio of the upstream and downstream connecting sections. The total head loss coefficient varies between 0.837 and 1.464. The maximum hydraulic gradient of total head occurs at the downstream culvert inlet and its connecting section, where the head loss accounts for 55.07% to 72.67% of the total head loss, and this section is the most sensitive to changes in bottom slope. Both excessively large and excessively small bottom slope ratios are prone to inducing unfavorable velocity patterns, thereby affecting the flow capacity of the culvert. The scheme with a bottom slope ratio of 1∶7 exhibits the minimum head loss, the optimal flow pattern, and excellent and stable hydraulic performance under different flow rates, with its flow coefficient ranging from 0.868 to 0.928.