A charging method, an authentication method, a charging device, and an authentication device. The charging method includes: receiving a service unit request sent by an access device, calculating a granted service unit according to a current tariff, determining a reporting time point according to a tariff switch point and returning a service unit answer to the access device; and receiving a used service unit reported by the access device at the reporting time point, calculating a remaining service unit based on the granted service unit and the used service unit instructing the access device to perform a forcible offline operation. The technical solutions provided by the embodiments of the present invention may effectively reduce an occurrence probability of a user account overdraft, thereby reducing a loss caused by the user account overdraft to an operator.