Embodiments of the present invention provide a system upgrade method and device, which can reduce a quantity and a time of service interruptions when an ONT and an OLT are upgraded. The method includes: detecting an upgrade triggering event, where the upgrade triggering event is used to instruct an optical network terminal ONT and an optical line terminal OLT to perform a concurrent upgrade; resetting the ONT to activate a preloaded ONT upgrade program; and restoring a service with the OLT. According to the embodiments of the present invention, an ONT and an OLT basically concurrently or synchronously perform upgrading and resetting, that is, a concurrent upgrade, so that a quantity and a time of service interruptions caused when the ONT and the OLT are upgraded are reduced, thereby improving user experience.