A passive optical network (PON) includes at least one optical network unit (ONU) at a downstream end and an optical line terminal (OLT) at an upstream end. Each ONU is dynamically allocated with an upstream communication window. During a laser always on (LAO) detection process, when the OLT receives an optical signal from the downstream end within an upstream idle window, the OLT determines that an LAO problem exists. The upstream idle window does not overlap with any upstream communication window. An anomaly detection method for the PON includes: during the LAO detection process, when the OLT receives an optical signal from the downstream end within the upstream idle window, determining, by the OLT, the LAO problem exists. An OLT includes an optical transceiver unit that optically communicates with the downstream end and output a receiving state signal and a control unit that determines whether the LAO problem exists.