A method for activating an Optical Network Unit (ONU) is provided, where the ONU communicates with an Optical Line Terminal (OLT) through a plurality of paths. The method includes: finding the ONU by the OLT on a path of the plurality of paths; and/or selecting one path from the plurality of paths and performs a ranging on the ONU on the selected path, by the OLT. Through this solution, the OLT finds the ONU and performs ranging on each ONU on only one of the plurality of paths, that is, does not activate each ONU on all paths, thereby quickly completing an activation process of a multi-path PON in a multi-path PON structure and improving work efficiency.