Provided is a method for registering an optical network unit. The method is used for an optical line terminal. The method for registering an optical network unit comprises: according to pre-acquired registration information of an optical network unit to be registered, issuing registration configuration information, so as to complete registration of said optical network unit, wherein the registration information of said optical network unit is acquired before the present registration starts. Further provided are a method for writing registration information, an optical line terminal, an optical network unit, a front-facing optical network unit, and a registration device.