A method for monitoring and controlling session traffic usage, characterized in that, comprising: receiving a data reading request sent by a first policy control function, wherein the data reading request is used for requesting to read user subscription information and traffic usage information of a user; sending the user subscription information and the traffic usage information of the user to the first policy control function; receiving total traffic usage information of a first session sent by the first policy control function, wherein a mode for accumulating the total traffic usage is set as that the first policy control function accumulates the total traffic usage; receiving a data reading request sent by a second policy control function, wherein the data reading request is used for requesting to read the user subscription information and the traffic usage information of the user; sending the user subscription information and the traffic usage information of the user to the second policy control function, and informing the second policy control function that the mode for accumulating the total traffic usage is that a user data repository accumulates the total traffic usage; informing the first policy control function to change the mode for accumulating the total traffic usage to that the user data repository accumulates the total traffic usage; receiving traffic usage increment information of the first session and traffic usage increment information of a second session reported by the first policy control function and the second policy control function; accumulating a total traffic usage of the first session and the second session according to the received traffic usage increment information; determining whether a current total traffic usage reaches a threshold value of a total issued traffic usage; and in response to that the current total traffic usage reaches the threshold value of the total issued traffic usage, sending total traffic usage information of the first session and the second session to the first policy control function and the second policy control function, so that the first policy control function and the second policy control function carry out policy control according to the total traffic usage information and the respective accumulated traffic usage increment information, and in response to that the current total traffic usage does not reach the threshold value of the total issued traffic usage, repeating above steps. receiving a data reading request sent by a first policy control function, wherein the data reading request is used for requesting to read user subscription information and traffic usage information of a user; sending the user subscription information and the traffic usage information of the user to the first policy control function; receiving total traffic usage information of a first session sent by the first policy control function, wherein a mode for accumulating the total traffic usage is set as that the first policy control function accumulates the total traffic usage; receiving a data reading request sent by a second policy control function, wherein the data reading request is used for requesting to read the user subscription information and the traffic usage information of the user; sending the user subscription information and the traffic usage information of the user to the second policy control function, and informing the second policy control function that the mode for accumulating the total traffic usage is that a user data repository accumulates the total traffic usage; informing the first policy control function to change the mode for accumulating the total traffic usage to that the user data repository accumulates the total traffic usage; receiving traffic usage increment information of the first session and traffic usage increment information of a second session reported by the first policy control function and the second policy control function; accumulating a total traffic usage of the first session and the second session according to the received traffic usage increment information; determining whether a current total traffic usage reaches a threshold value of a total issued traffic usage; and in response to that the current total traffic usage reaches the threshold value of the total issued traffic usage, sending total traffic usage information of the first session and the second session to the first policy control function and the second policy control function, so that the first policy control function and the second policy control function carry out policy control according to the total traffic usage information and the respective accumulated traffic usage increment information, and in response to that the current total traffic usage does not reach the threshold value of the total issued traffic usage, repeating above steps.