A data processing method, where a first device communicates with a second device using a relay node, where the relay node does not have a Packet Data Convergence Protocol (PDCP) entity, and the method includes transmitting, by the first device, a first data unit group, where the first data unit group is in a first sequence range, receiving, by the first device, first information using the relay node, where the first information indicates a data unit that the second device has received or has not received in the first data unit group, determining, by the first device, a second sequence range based on the first information.