A measurement method, a device, and a system are provided, so that a communications device (40) can perform criterion S determining on a serving cell when performing RRM measurement on a non-anchor carrier. The method includes: A communications device (40) determines a first reference signal received power based on at least one measurement result obtained by measuring a reference signal received power on a first non-anchor carrier (S502), where the first non-anchor carrier is a carrier on which the communications device (40) receives a paging message from a network device (30) in a serving cell in which the communications device (40) is located; and the communications device (40) performs criterion S determining on the serving cell based on the first reference signal received power (S503).