Embodiments of this application provide key update methods and apparatuses in the field of communications technologies. A communications system includes a terminal and a core network device. The terminal can access the core network device using both a first access technology and a second access technology. The first connection and the second connection have a shared key. Key update for the first connection is performed in obtaining a first key identifier that identifies a first key obtained by performing the key update for the first connection. In response to determining that the second connection is in a connected state, the shared key for the second connection and a second key identifier that identifies the shared key are retained. The shared key is kept using for the second connection before performing key update for the second connection.