The present disclosure relates to the field of communications technologies, and discloses a channel training method, apparatus, and system, so as to resolve a problem in which a newly added ONU in a PON cannot be registered and go online in time. In embodiments of the present disclosure, a first moment for triggering channel training is determined; normal data is stopped sending from the first moment and a training frame is generated; and then the training frame is sent to all ONUs in a PON, so that a target ONU trains an automatic adaptive equalizer based on the training frame, where the target ONU is at least one of all the ONUs in the PON. The solutions provided in the embodiments of the present disclosure are applicable to the equalizer training the ONU.