The present invention discloses a method for providing an online-charging to solve a problem that a related charging can not be processed correctly for a service involving simultaneously a CS domain and an IMS. The method includes: not invoking an online charging for the user in the CS domain when the user, who subscribes for the service involving simultaneously the CS domain and the IMS and the online charging service, originates or terminates a call in the CS domain; and performing the credit control in the CS domain and/or the IMS for the user in the IMS when the call of the user is processed through the IMS. The present invention also discloses a device and a system for an online credit control.