This application provides a mapping relationship updating method, applied to deterministic networking. The method includes: A first device sequentially obtains a first packet and a second packet that are sent by a second device; the first device determines a first time difference between a time at which the first packet is received and a time at which the second packet is received; and if the first device determines that the first time difference is not within a preset range, the first device updates a period-based mapping relationship.