A water supply device put on the soil or buried in the soil in which the plants are planted. The water supply device includes a first part, a second part and a pipe. The first part is an annular part with a notch. The first part has a water storage space for storing water therein. The second part is located within the notch of the first part and has a room for storing fertilizer therein. The pipe is arranged along the first part and connected between the water storage space and the room. The pipe has a plurality of apertures distributed over the wall thereof. By using principles of differential-pressure and siphon, the water and fertilizer automatically move and are supplied to the plants so as to save time, expenses and efforts.
