A lighting-system control method applied to the lighting-system including a light source and an ultrasonic transmitting-and-receiving device is provided. The method comprises steps of: determining a distance between an object and the ultrasonic transmitting-and-receiving device by the ultrasonic transmitting-and-receiving device according to a flight time of an ultrasonic-wave; changing a first light property of the light source when the object is detected moving toward the ultrasonic transmitting-and-receiving device; and changing a second light property of the light source when the object is detected moving away from the ultrasonic transmitting-and-receiving device.