The present invention relates to the field of communications technologies, and in particular, to an online charging method, apparatus, and system based on a number portability service. When a calling UE in an IMS network initiates a call to a called UE, after receiving an invite message sent by a CSCF entity in the IMS network, an AS in the IMS network queries number portability information corresponding to a number of the called UE and sends a number of the calling UE and the found number portability information to an OCS in the IMS network, so that the OCS is capable of performing charging authentication for the calling UE according to the number portability information and the number of the calling UE. When the called UE has changed a subscription network and subscribed to an NP service, the OCS is thus capable of correctly performing charging authentication.