The present invention discloses a SIM substitution method, device and system, and belongs to the mobile phone design field. The SIM substitution method comprises the steps of connecting a mobile device, after connecting the mobile device successfully, receiving an obtaining request sent by the mobile device and used for requiring to obtain the information of a SIM; utilizing a communication converter to read the information of the SIM; utilizing the communication converter to send the read information of the SIM to the mobile device. The SIM is arranged inside an external device independently, by the information conversion, the information of the SIM is read to an information memory of the SIM, and in the mobile device, the information memory of the SIM substitutes for the SIM and a SIM card slot, thereby solving the problem that the SIM occupies the area of an arrangement board, and realizing the effects of reducing the PCB area of a mobile phone mainboard, and being convenient to replace cards.