The invention belongs to the field of milk cow breeding, particularly relates to a constant temperature watering device for a milk cow. The constant temperature watering device comprises a water inlet tube, a heat exchange tube coiling back and forth in a zigzag manner, a water outlet tube and a water trough, wherein the water outlet tube comprises a water inlet end and a water outlet end; the heat exchange tube is positioned at a deep position 7-10 meters below the ground surface; one end of the heat exchange tube is in sealed connection with the water inlet tube, and the other end of the heat exchange tube is in sealed connection with the water inlet end; a floating block capable of floating in tap water and a limiting frame for limiting the floating block to only float up and down are arranged in the water trough; the limiting frame comprises four vertical columns fixed at the bottom of the water trough; the floating block is positioned in the limiting frame; the floating block is in sliding connection with the vertical columns; the water outlet end is positioned in the limiting frame and at a position above the floating block. The constant temperature watering device for the milk cow can ensure suitable temperature and smaller work amount.