A liquid supply device adapted to a liquid cooling system includes a casing, a cover, a plunger, a driving unit and a sensing unit. The casing has a liquid outlet. The cover is connected to the casing. A chamber is formed between the casing and the cover and contains a cooling liquid. The chamber communicates with the liquid outlet. The plunger is movably disposed in the chamber. The driving unit is disposed in the chamber and used for driving the plunger to move. The sensing unit is selectively disposed on one of the casing and the plunger. The sensing unit senses a liquid level of the cooling liquid and outputs a sensing signal with a predetermined status as the liquid level is equal to or lower than a predetermined height.