An automatic liquid supply apparatus for supplying liquid material during 3D printing is provided. The automatic liquid supply apparatus includes: a container including an accommodating space for containing the liquid material, the accommodating space including a recess and a trench communicating with each other, the trench having an inner bottom face, the recess being recessed to a depth below the inner bottom face; a liquid level sensor including a floater, the floater being disposed corresponding to the recess and floating to rise and fall along with a liquid level, the liquid level sensor sensing the liquid level in the accommodating space; and a liquid dispenser electrically connected to the liquid level sensor, the liquid dispenser selectively supplying or stopping supplying the liquid material into the accommodating space according to the sensed liquid level.