Disclosed is a water cooling system for a circuit board with heat generating components. The water cooling system includes a fin module, a water block, a liquid guiding tube and a fan. The fin module is installed on the circuit board and has a through hole; the water block is clamped between the fin module and the circuit board and has a side thermally coupled to the heat generating components, and further has a liquid inlet end and a liquid outlet end; the liquid guiding tube communicates with the liquid inlet end and the liquid outlet end and passes and is fixed into the through hole and thermally coupled to an inner wall of the through hole; and the fan is configured to be corresponsive to the fin module. Therefore, the water cooling system has the advantages of reduced volume and high cooling efficiency.