Embodiments of the present invention disclose a data transmission method and apparatus. The data transmission method includes: performing, by a Packet Data Convergence Protocol PDCP entity, lossless transmission on PDCP protocol data units PDUs or PDCP service data units SDUs, where each of the PDCP PDUs or the SDUs is associated with one serial number. In this embodiment, during lossless transmission, no data packet is lost, thereby ensuring communication quality in an inter-station carrier aggregation scenario and achieving an optimization objective.