The invention discloses an online charging engine, a real-time charge off engine, and an online charging method and system. The online charging method comprises the following steps: receiving an online charging request, obtaining current user state information corresponding to a terminal identifier of a terminal, and performing authentication on a service request according to the current user state information, wherein the current user state information is user state information after the online charging processing of the previous online charging request of the terminal is terminated, and an authentication result of the service request is sent to a service network element. By adoption of the online charging method, service authentication and real-time charge off are asynchronous processing procedures, the response period of the service authentication is shortened, the retention rate of the online charging flow of the user is improved, the working load of the online charging system on service peaks is reduced, and thus the technical problem of large working load of the online charging system on service peaks, relatively low retention rate of the online charging flow of the user and influence to the service perception of the user on the flow service in the prior art are solved.