The present disclosure provides a data transmission method, related devices and system. The method includes: an upper layer transmits a data packet to a corresponding Service Data Adaptation Protocol (SDAP) entity, according to at least one of a network slice identifier, an SDAP identifier, a flow identifier or a session identifier; the SDAP entity adds the flow identifier to the data packet, and transmits the data packet with the flow identifier to a Packet Data Convergence Protocol (PDCP) entity corresponding to a Data Radio Bearer (DRB), according to a corresponding relationship between the SDAP entity and the DRB.