A data exchange device that identifies VoIP traffic flow. When a type of data flow is received by the data exchange device, the data exchange device determines whether the type of data flow is bidirectional traffic flow. If the type of data flow is bidirectional traffic flow, the data exchange calculates a first data size of downstream data of the type of data flow and a second data size of upstream data of the type of data flow. Then, the data exchange device identifies whether the type of data flow is the VoIP traffic flow according to the first data size and the second data size. If the type of data flow is the VoIP traffic flow, the type of data flow is processed according to a high priority.