Embodiments of the present disclosure provide an activation method, an activation device, a control device, a network device and an optical network system. A new-system ONU is activated on a first OLT in an existing system, and then activation information of the new-system ONU is sent to a second OLT in a new system by the first OLT or by an activation agent ONU which has been activated on the second OLT, thereby enabling activation of the new-system ONU on the second OLT.