Embodiments of the present invention provide a link switching method, device, and system. The method comprises: an optical network unit (ONU) transmits a monitoring packet through a first Maintenance association End Point on a first link connected to the ONU and an aggregation side switch device, so as to monitor the first link; the ONU performs switching from a first sub-link of the first link to a first sub-link of a second link if it is detected that a link fault occurs on the first link; the ONU sends a notification message to a second optical line terminal (OLT), so that the second OLT enables a transmission port; and the ONU sends an automatic protection switching message to the aggregation side switch device through a second Maintenance association End Point on the second link.