Embodiments of this application provide a device-to-device based resource determining method and device, which may be applied to a D2D system (for example, a system such as an internet of vehicles, V2X, or V2V). The method includes: receiving first information sent by a first terminal device, where the first information indicates a second terminal device to send assistance information; and sending the assistance information to the first terminal device, where the assistance information is used to assist the first terminal device in selecting a resource for data transmission. The second terminal device assists the first terminal device in determining the resource for data transmission, instead of completely determining, by a terminal device that sends data, the resource for data transmission.