A device-to-device (D2D) data transmission method and system, where the method implemented by a source terminal and includes sending a first message to n candidate terminals, where the first message discovers the n candidate terminals, the source terminal is out of a network coverage area of a base station, and the n candidate terminals are in the network coverage area of the base station, receiving n second messages from each candidate terminal, wherein each second message includes at least one of a user identity of each candidate terminal or status information of each candidate terminal, determining a relay terminal from the n candidate terminals according to at least one of the user identity of each candidate terminal or the status information of each candidate terminal, and transmitting the data to the base station using the relay terminal.