This disclosure relates to a data transmission method, apparatus, and system, and belongs to the field of communications technologies. The data transmission method includes: obtaining, by a first node, m (m≥2) first FlexE clients, mapping the m first FlexE clients to one second FlexE client, and transmitting data of the second FlexE client to a second node, where data of each first FlexE client occupies a fixed timeslot of the second FlexE client, and the second node is different from the first node. In this disclosure, a problem in a related technology that a relatively fine granularity of a FlexE client imposes a relatively strict requirement on data processing performance of a node is resolved, and a plurality of first FlexE clients are mapped to one second FlexE client.