Embodiments of this application provide an information transmission method, an optical line termination, an optical network unit, and a communications system. A method on an optical line termination OLT side includes: allocating, by the OLT, an identifier to a first optical network unit ONU through a first channel, and performing, through the first channel, ranging on the first ONU to obtain ranging information about the first channel; and after determining, by the OLT and the ONU through negotiation, to use two channels to perform information transmission, performing, by the OLT, data transmission of a first service with the first ONU through a second channel. In this method, identifier allocation and ranging are performed through the first channel, and the data transmission of the first service is performed through the second channel. In this way, a relatively large delay is no longer generated for service data that has a relatively high requirement for delay. This further ensures normal execution of these services.