The invention provides a method and a system for implementing the flow sharing of multiple mobile terminal cards. The method concretely comprises the following steps: an additional data card platform is established; E. 214 route data are configured for an HSTP (high signaling transfer point), and a special IMSI (international mobile subscriber identity) number segment route for an additional data card is directed to the additional data card platform; the additional data card platform participates in the PS (packet switched) domain attachment process of the additional data card, so that the additional card generates Internet call tickets taking a main card MSISDN (mobile subscriber ISDN (integrated services digital network) number) and an additional card IMSI as identifiers in an SGSN (serving GPRS support node) and a GGSN (gateway GPRS (general packet radio service) support node); and after acquiring a billing center, the call tickets are combined through taking MSISDN as an identifier, so that the Internet call ticket of the additional card is merged into an account of the main card. According to the invention, through providing the additional data card platform, the flow sharing of multiple mobile terminal cards is implemented through the signaling exchange between the additional data card platform and a core network, no improvement on the core network, a BSS (base station subsystem) and a CRM system is required, the service processes of the existing BSS, SGSN (service GPRS supporting node) and CRM and the like are not affected, and no influence is caused on an operator network, thereby realizing the inter-flow sharing of 3G main number packages.