This application provides an OLT, an ONU, a PON system, and an information transmission method in a PON system. The method includes: sending, by an OLT, first information to an unregistered ONU, where the first information includes at least one piece of indication information, and one piece of indication information indicates a first power range and a first time range associated with the first power range; and receiving, by the OLT in the first time range, a registration message sent by the ONU, where a downstream receive power of the ONU falls within the first power range. Registration signals of ONUs whose downstream receive powers fall within a same power range are similar or differ little in signal strength when reaching the OLT, so that the OLT can quickly respond to a registration signal of an ONU, and an overall response time of the OLT for registration signals of different ONUs can be shortened.