A data sending method, a terminal device, an access network device, and a system are provided. The method includes: obtaining, by a terminal device, a sending period of first data; obtaining, by the terminal device, configuration information of a first resource; determining, a second resource based on the configuration information of the first resource and a sending period of the first data; and selecting, by the terminal device, a resource other than the second resource to send the first data. When selecting a resource used for V2X communications data or D2D communications data, the terminal device excludes the second resource that cannot be used for the V2X communications data or the D2D communications data, so as to avoid a conflict between the second resource and the resource selected by the terminal device for sending the first data, and improve reliability and security of D2D communication and V2X communication.