The invention discloses a method, a system and a device for detecting faults of light distribution networks, wherein the method comprises the following steps that: the losses of the uplink and downlink signals on the side of OLT and the side of ONU/ONT are respectively detected; the optical fiber bending fault can be determined if the loss of the downlink signal is more than that of the uplink signal; the connector fault can be determined if the losses of the uplink and the downlink signals are equal; the system comprises a first signal detection module arranged on the side of OLT and a second signal detection module arranged on the side of ONU/ONT, which are used for respectively detecting the optical power of the uplink and the downlink signals on both sides; a loss calculation module which is used for calculating the losses of the uplink and the downlink signals; and a fault analysis module which is used for analyzing and determining the type and the position of the faults according to the losses of the uplink and the downlink signals. The method, the system and the device can conveniently detect the type and the position of the faults without special requirements to the optical fiber distribution, and are suitable for online and real-time fault detection.