A vehicle-mounted device upgrade method and a related apparatus, where the method includes an on-board unit (Tbox) of a vehicle that processes a first data segment according to a first algorithm to obtain a first check value, where the first data segment is any data segment in a plurality of data segments included in an upgrade file of a control unit, and the first check value is sent to the control unit, and the Tbox encrypts the first data segment by using a first key to obtain a first encrypted segment, and sends the first encrypted segment to the control unit, so that the control unit stores, in the control unit, the first data segment obtained by decrypting the first encrypted segment by using the first key, where the first data segment is used by the control unit to form the upgrade file for upgrade.