Embodiments of this application provide a relay communication method, apparatus, and system. In the method, when a first terminal device communicates with a second terminal device through a relay device, the first terminal device sends an RB configuration to the second terminal device by using the relay device. The RB configuration includes the first identifier and a PDCP entity configuration; or the RB configuration includes the PDCP entity configuration, and the PDCP entity configuration includes the first identifier. According to the method in this application, the first terminal device and the second terminal device may perform data transmission by using the same security protection parameter, to ensure correctness and effectiveness of data security protection in a process in which the first terminal device communicates with the second terminal device through the relay device.