In view of the fact that the international standard for passive optical network（PON） has not specified the method of remote software image upgrade for the composite optical network unit（ONU） system, a remote software image upgrade method for composite ONU is designed. The method expands upon the international standard while meeting its requirements, including the design of a composite image file format that can contain multiple image files and the method of creating such files, as well as the development of an ethernet-based out-of-band management and file transfer protocol （used for efficient transmission of image files）. At the same time, the method is compatible with a variety of international/domestic standard protocols. The experimental results show that show that the proposed remote upgrade method is functionally effective and highly efficient.