The present invention discloses a data transmission system. The first device performs, at a cellular network PDCP layer of the first device, security processing on to-be-transmitted data based on a transmit end-based first policy, to generate a PDU, and sends the PDU to the relay UE, where the first policy is a security processing policy agreed on by the first device and the second device, and the to-be-transmitted data is data that the first device needs to transmit to the second device. The relay UE sends the PDU to the second device. The second device performs, at a cellular network PDCP layer of the second device, security processing on the PDU based on a receive end-based first policy, to obtain the to-be-transmitted data, where the cellular network PDCP layer of the first device is a peer to the cellular network PDCP layer of the second device.