The ultimate bearing capacity and failure modes of the maintenance gate slot specimens with different reinforcement ratios were studied through static load tests of three scaled specimens. Based on ABAQUS, numerical simulation of the prototype of the Laxiwa Dam maintenance gate slot was conducted after the numerical model was validated by the experimental results. The failure modes and load transfer mechanism of the maintenance gate slot with different shear span ratios and reinforcement ratios were investigated. The results indicate that the shear capacity of the gate slot is higher than the local compressive bearing capacity if the second-stage concrete is cast without strengthening detailing. Provided that there is sufficient local bearing capacity, the gate slot mainly fails in shear due to the failure of the concrete compression strut if the shear span ratio is greater than 0. 18, otherwise the local sliding damage is dominated. Adding the layers of the horizontal reinforcement along the water flow direction in the maintenance gate slot is able to effectively improve the shear bearing capacity.