Disclosed are a charging method, a network device and a system, for use in recovering an online charging function in time when an OCS is faulty, thereby reducing risks of user account overdraft, and improving user experience. The method in an embodiment of the present invention comprises: a PCEF entity performs online charging on a UE; when it is known that an online charging system (OCS) is unavailable, the PCEF entity switches the UE from online charging to offline charging, and starts an offline timer; and if the PCEF entity knows, before the offline timer times out, that the OCS is recovered to be available, the PCEF entity switches the UE from the offline charging to the online charging. In the solution, after users are switched to offline charging, an online charging function is recovered in time when an OCS is available, thereby reducing possible risks of user account overdraft during an offline charging process; and the solution can avoid deactivating users, thereby improving user experience.