A charging method for a Push-to-Talk over Cellular, PoC, system, comprising: receiving, by a PoC server, a service initialization request sent by a PoC client, wherein the service initialization request comprises a Support-Multicast-Channel Indication indicating that the PoC client supports a multicast channel; sending, by the PoC server, an initial online charging request to an Online Charging System, OCS, wherein the initial online charging request comprises the Support-Multicast-Channel Indication and a type of a downlink channel currently used by a PoC service; receiving, by the PoC server, an initial online charging response message that is sent by the OCS and carries a downlink channel type switching trigger variable, wherein the downlink channel type switching trigger variable is determined by the OCS according to the Support-Multicast-Channel Indication, wherein the initial online charging response message further comprises a Granted Service Unit, GSU, that is reserved by the OCS according to the type of the downlink channel currently used by the PoC service; monitoring, by the PoC server, the type of the downlink channel used by the PoC service according to the downlink channel type switching trigger variable and monitoring usage of the reserved GSU by the PoC service according to the reserved GSU; and sending, by the PoC server, an online charging request to the OCS if switching of the type of the downlink channel used by the PoC service is detected, wherein the online charging request comprises a switched type of the downlink channel; wherein the online charging request further comprises a timestamp at the time of switching the type of the downlink channel used by the PoC service; and the timestamp is used for the OCS to generate a user bill and/or calculate duration of using a downlink radio channel by the PoC service. receiving, by a PoC server, a service initialization request sent by a PoC client, wherein the service initialization request comprises a Support-Multicast-Channel Indication indicating that the PoC client supports a multicast channel; sending, by the PoC server, an initial online charging request to an Online Charging System, OCS, wherein the initial online charging request comprises the Support-Multicast-Channel Indication and a type of a downlink channel currently used by a PoC service; receiving, by the PoC server, an initial online charging response message that is sent by the OCS and carries a downlink channel type switching trigger variable, wherein the downlink channel type switching trigger variable is determined by the OCS according to the Support-Multicast-Channel Indication, wherein the initial online charging response message further comprises a Granted Service Unit, GSU, that is reserved by the OCS according to the type of the downlink channel currently used by the PoC service; monitoring, by the PoC server, the type of the downlink channel used by the PoC service according to the downlink channel type switching trigger variable and monitoring usage of the reserved GSU by the PoC service according to the reserved GSU; and sending, by the PoC server, an online charging request to the OCS if switching of the type of the downlink channel used by the PoC service is detected, wherein the online charging request comprises a switched type of the downlink channel; wherein the online charging request further comprises a timestamp at the time of switching the type of the downlink channel used by the PoC service; and the timestamp is used for the OCS to generate a user bill and/or calculate duration of using a downlink radio channel by the PoC service.