This application provides a method for protecting a truncated parameter and an apparatus, and relates to the field of communications technologies, to ensure security of a truncated parameter in a transmission process. The method includes the following steps: A mobility management network element determines whether a terminal that accesses a network meets a preset condition, where the preset condition includes that the terminal uses a control plane CIoT 5GS optimization function. The mobility management network element sends, to the terminal when the terminal meets the preset condition, a downlink NAS message on which NAS security protection is performed by using a NAS security context, where the downlink NAS message includes a truncated parameter. This application is applicable to a truncated parameter transmission process.