The present invention is directed to control method for supplying water in a humidifier. In a conventional humidifier, when the water level in the base of the humidifier is lowered to the ON / OFF critical region, with the vibration of the water level, the floating ball in the base will make ten's or hundred's times of ON / OFF vibrations, which tends to damage the high frequency vibration circuit. In addition, when the humidifier is inclined, the magnet on the floating ball remains in the attractive state with the magnetic spring switch that may cause fire of the electrical wires and leakage of the electricity. In the present invention, the floating ball is directly placed within a water tank, so that the magnetic spring switch can be effectively driven to conduct the ON / OFF movement, and the power can be cut off immediately when the water level of the water tank is lowered to the minimum level. When the humidifier is turned over by external force, the water tank will become separated from the base of the humidifier, which will break the contact between the floating ball and the magnetic spring switch so as to cut off the power, assuring the safety and extending the use time of the humidifier.