This application provides a communication method, including: receiving, by a terminal device, configuration information, where the configuration information is used to configure M component carriers, and M is a positive integer; determining, by the terminal device, N types of DCI sizes based on the configuration information; and if N is greater than a first threshold, determining, by the terminal device, Q types of DCI sizes, and detecting a downlink control channel in a first time unit based on the Q types of DCI sizes, where Q is less than or equal to the first threshold. According to the communication method provided in this application, a capability of detecting a DCI size by a terminal device can be fully used.