A data flow classification device includes a forwarding circuit and a configuring circuit. The forwarding circuit looks the classification of an input flow up in a lookup table according to the information of the input flow, tags the packets of the input flow with the classification, and outputs the packets to a buffer circuit; but if the classification is not found in the lookup table, the forwarding circuit tags the packets with a predetermined classification, outputs the packets to the buffer circuit, and adds the information of the input flow to the lookup table. The configuring circuit determines a flow threshold according to a queue length of the buffer circuit and a target length, learns the traffic of multiple flows from the lookup table, determines the classifications of the multiple flows according to the comparison between the traffic and the flow threshold, and stores these classifications in the lookup table.