The embodiment of the present disclosure relates to a data transmission method and a terminal device. The method includes if a terminal device determines, according to first configuration information, to perform a first type of transmission on a target transmission resource, and determines, according to second configuration information, to perform a second type of transmission on the target transmission resource, the terminal device determines whether the first type of transmission or the second type of transmission is a target type of transmission according to a first preset rule, wherein the first type of transmission is sidelink transmission between the terminal device and another terminal device, and the second type of transmission is uplink transmission or downlink transmission between the terminal device and a network device; and performing data transmission on the target transmission resource by using the target type of transmission.