The invention relates to a method and a device for classifying network traffic on line based on decision tree high-speed parallel processing. The method comprises the following steps of: performing acquisition, distribution and manual classification on early real traffic data, extracting the packet characteristics of an early transmission control protocol (TCP) stream set, establishing a decision tree classification model, converting a data structure, performing distribution and class judgment on a data packet to be classified, tagging a current data packet, extracting the packet characteristics of a TCP stream to be classified, and searching for a decision tree. The device comprises a decision tree construction module, a structure conversion module, a classification result processing module, a medium access control (MAC) layer processing module, a data packet polling management module, a distribution judgment module, a traffic information extraction and tagging module, and a decision tree searching module. The method and the device are low in algorithm complexity and high in processing speed, classification accuracy and stability, and can be used for equipment and systems with requirements for online traffic classification in a high speed backbone network; and online classification can be realized.