In the aim of investigating the drainage effect of horizontal drainage holes in bank slope, in-situ tests were carried out on a bank slope of Yangtze River through arranging horizontal drainage holes of different depths with replaceable filter. The drainage effects were compared with that of blind ditch to validate the effectiveness and operability of drainage holes. Results of water level monitoring suggest that the water level in bank slope is closely related with that of the Yangtze River when river water level rises, groundwater of bank slope is recharged; whereas when river water level declines, groundwater of bank slope discharges to the river. We conclude that horizontal drainage holes have good effect in drainage and pressure reduction. The drainage effect of bank slope in the presence of drainage holes is better than that of conventional blind ditch; and the effect of drainage holes of 15m depth is superior to that of 8m depth.