The invention discloses a method and system for obtaining the cell phone number of a 3G network user. The method includes the steps that a mobile terminal interacts with an SGSN/GGSN through a 3G network to complete PDP activation, and an accounting gateway arranged in a CGN sets up the mapping relation between the cell phone number of the user and a private network IP address of the mobile terminal; an SP network inquires of an query server where a legal SP website is stored so as to obtain a URL of the accounting gateway; the SP website sends redirection information to the mobile terminal, wherein the redirection information includes the URL of the accounting gateway and the address of a rebound SP service website; the mobile terminal obtains the cell phone number from the accounting gateway according to the URL of the accounting gateway in the redirection information and requests for having access to the SP website when carrying the cell phone number; and the SP website obtains the cell phone number according to the request of the mobile terminal. The method and the system can ensure that the SP website can inquire the cell phone number of the user from the accounting gateway.