An automatic sensing system and a detecting method for use with plumbing equipment in the kitchen and washroom are shown and described. The automatic sensing system includes an ambient light sensor for detecting the ambient light intensity around the equipment. The system further includes a detecting sensor for detecting whether user enters the detected area. The system also includes a controller which controls the equipment based on detected signals from the detecting sensor. The controller is configured to start or change the state of the detecting sensor according to a detected value change of the ambient light intensity. The automatic sensing system advantageously saves power consumed by the detecting sensor when the ambient light sensor does not indicate use by the equipment.