A timer processing method includes: receiving, by a terminal device, a first message sent by a network device, and starting or restarting a timer based on a scrambling identifier of the first message, wherein the timer is a timer used by the terminal device to switch from an active downlink BWP to a default downlink BWP, or is a timer used by the terminal device to activate a default downlink BWP and deactivate an active downlink BWP; or if an active BWP pair of the terminal device is not a default BWP pair, starting or restarting, by the terminal device, a timer based on a scrambling identifier of the first message, wherein the timer is a timer used by the terminal device to switch from the active BWP pair to the default BWP pair, or is a timer used by the terminal device to activate the default BWP pair.