The present invention discloses a method, an apparatus, and a system for transmitting data. A method for sending data includes: categorizing a received packet based on field information of a preset field in a packet header; encapsulating, for each category of packets, field information of a preset field in a packet header of a packet to an overhead field of a data unit, and encapsulating a packet payload of each packet and field information of another field except the preset field in a packet header of each packet to a payload field of the data unit; and sending the generated data unit to a destination node of the packet. The method, the apparatus, and the system are capable of improving bandwidth usage of a transmission link.