A liquid supply device 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. 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 cover and the plunger. The sensing unit outputs a sensing signal according to a current position of the plunger.