A water-cooled heat sink includes a base, a box, a propelling module, a driving module, and a plurality of conduits. The box is disposed above the base, and receives a cooled liquid. The driving module includes a first magnet. The base defines a plurality of channels. The propelling module includes a cylinder, a piston, a second magnet, and a valve. The cylinder is disposed on an inner wall of the box. The piston is disposed in the cylinder, and defines a through hole in a middle portion thereof. The second magnet is fixed to the piston. The valve is fixed in the through hole. The conduits interconnect the cylinder and the box. The first magnet can repel or attract the second magnet, thereby sliding the piston toward the cylinder to close the valve or away from the cylinder to open the valve.