Disclosed is a data flow classification device including 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. The configuring circuit receives and stores the identification and traffic information of multiple flows, and accordingly calculates the traffic of the multiple flows, wherein the multiple flows include the input flow. The configuring circuit further determines an elephant flow threshold according to a queue length of the buffer circuit and a target length, determines the classifications of the multiple flows according to the comparison between the traffic of the multiple flows and the elephant flow threshold, and stores these classifications in the lookup table.