A method and apparatus for authentication in a passive optical network are disclosed. In the disclosure, a PLOAM message is sent from an ONU to an OLT, where the PLOAM message includes a first field for carrying an ONU identifier (ONU-ID) and a second field for carrying a first logic registration code, where the ONU-ID is assigned by the OLT to identify the ONU and wherein the first logic registration code is assigned to a user by an operation management system communicatively connected to the OLT and provisioned to the user for authentication. The OLT is configured to judge whether the first logic registration code received from the ONU matches with a second logic registration code provisioned by the operation management system to the OLT and to promote the ONU into service if the first logic registration code matches with the second logic registration code.