Based on tests of a rectangular one-span simply supported aqueduct, the tuned liquid damper (TLD) effect on the aqueduct under the condition of a rigid foundation subjected to strong vibration was analyzed. The relationship between the transverse damping effect and the ratio of depth to width were studied by adjusting the water level. The test results demonstrated that water sloshing and wave breaking had a TLD effect on the aqueduct structure; the increase of the water level in the aqueduct changed the dynamic properties of the structure, causing the self-vibration frequency of the structure to decrease; during strong vibration, the TLD effect fluctuated with the change of water level; and the most significant TLD effect occurred when the water sloshing frequency was close to the self-vibration frequency of the structure.