The invention discloses a method and system for building a highly reliable mobile cash register system, which adopts three cash register modes that the normal running line is line one, and two groupsof standby lines are line two and line three respectively, wherein the line one is from a customer, a cash register, a store network, a fourth party payment server to a third party payment server; theline 2 is from the customer, the cashier, the internet of things, the fourth party payment server to the third party payment server; the line 3 is from the customer, the cashier, the 4G network to the third party payment server. Through the design of line 2, the mobile cash register business can be kept uninterrupted under the condition of the outage of the store. Through the design of the line 3, and when the fourth-party server is down, the store will exchange information with the third-party payment server directly, and synchronize the information with the fourth-party payment server afterthe fourth-party payment server is restored. The online rate of mobile cash register service can be approximated to the official online rate of the third-party payment server by this scheme.