The present disclosure relates to information transmission methods in a passive optical network (PON) system. One example method includes sending, by an optical line terminal (OLT), a first power range and time indication to an unregistered optical network unit (ONU), where the first power range and the time indication indicate the ONU to send a serial number of the ONU to the OLT at a time indicated by the time indication in case a downlink receive power of the ONU is within the first power range, and receiving, by the OLT, the serial number of the ONU.