This application provides a data transmission method and an apparatus. The method includes that an integrated access and backhaul (IAB) donor distributed unit receives downlink data and service attribute information of downlink data that are sent by an IAB donor centralized unit. The IAB donor distributed unit maps the downlink data to a corresponding backhaul radio link control channel (BH RLC) channel based on the service attribute information of the downlink data or based on the service attribute information of the downlink data and access IAB node address information of the downlink data, and sends the downlink data to a next-hop IAB node.