The present disclosure relates to an information writing method and device, and belongs to the technical field of communication. The method includes that: code number information of a Subscriber Identification Module (SIM) card is acquired, the code number information being verification information during access of the SIM card to a network of an operator; the code number information is sent to a Card Operating System (COS) of the SIM card through an original interface in an operating system, the original interface being a communication interface programmed for transmission of other information between the operating system and the COS; and the code number information is written into the SIM card through the COS.