This application provides a data transmission method and apparatus, to improve data transmission reliability. The method includes: receiving, by a first terminal device, first indication information sent by a network device, where the first indication information is used to indicate a first preset condition, and the first preset condition is used to determine a resource used for sending data; determining, by the first terminal device from a first resource set according to the first indication information, a first resource that meets the first preset condition; and sending, by the first terminal device, first data by using the first resource.