The invention discloses a water-power-driven automatic water drainage system for a hilly-area farmland. The water-power-driven automatic water drainage system comprises a buoy slot, a buoy, a straightrod, a latch connecting structure, copper sheet plugs, a water drainage tube, a water inlet, a through hole, a floating barrel, a rope and a gate, wherein water level exceeds needed water level; thebuoy rises, and the straight rod which is connected with the buoy also rises to drive the copper sheet plug on the upper part to move upwards to block the through hole; and the copper sheet plug on the lower part moves upwards, the water drainage tube is opened, water level of a vertical shaft drops, and correspondingly the gate rises and opens to drain water. Compared with the prior art, the device can timely and effectively drain too much ponding in the farmland while rainfall is too much or the farmland is invaded by a great deal of exogenous water, so that crops are prevented from being flooded to form a water logging disaster. Meanwhile, the device can manually set farmland water layer control depth according to a crop condition, and does not need manual duty, so that water drainage efficiency is greatly improved. The water-power-driven automatic water drainage system is reasonable in design, is high in practicability and is convenient for popularization and application.