The present disclosure provides a ranging method and a registration method for an optical network, an Optical Line Terminal (OLT), an Optical Network Unit (ONU), and an optical network system. An OLT groups ONUs controlled by the OLT according to receiving optical power values of the ONUs. The OLT allocates corresponding uplink bandwidths to the ONUs in different groups, the uplink bandwidths being used to send registration information by the ONUs to be registered.