A water-replenishing and gas-removing structure for water cooling device includes a flow-guiding main body having a water-receiving space as well as an inlet, an outlet, a first opening and a second opening, which are communicable with the water-receiving space. The first and the second valve member are located in the water-receiving space corresponding to the first and the second opening, respectively, for opening or closing them. The first and the second connecting member are connected at an end to the first and the second opening, respectively, and at another end to an external water-replenishing and an external gas-removing apparatus, respectively, via a pipe each. With these arrangements, cooling fluid can be replenished into and surplus gas can be removed from a water cooling device without the need of disassembling or reworking the water cooling device.