The invention discloses methods for determining and controlling an online charging user, and a system and a device for controlling the online charging user. The main technical scheme is that the method for determining the online charging user comprises the following steps of: receiving a calling request by a mobile switching center (MSC) from a first user to a second user; acquiring number information comprised in the calling request, wherein the number information is a communication number corresponding to the second user or a number prefix of the communication number, which corresponds to the second user; determining whether the acquired number information is online charging number information according to the stored online charging number information; and if the acquired number information is the online charging number information, determining that the first user is subjected to online charging processing. According to the technical scheme, after the user sends the calling request, whether the user is subjected to online charging processing is determined in real time according to the number information acquired from the calling request sent by the user, so that the user with the debt risk can be controlled in real time.