Loop detecting and controlling method

The invention provides a loop detecting and controlling method. The method comprises the following steps that: an optical network unit (ONU) periodically sends a loop detection message to the port of each user side according to the preset message issuing period, wherein the port of the user side is the port of each user side in all virtual local area network of the ONU; the ONU monitors whether the loop detection message is currently received or not according to a real-time or periodical way, if so, it is judged that a loop exists currently, and the loop is controlled in a way of alarming to an optical network terminal (OLT) and closing the port of the user side for receiving the loop detection message. The invention is suitable for an EPON (Ethernet Passive Optical Network) system.