The invention relates to a flat single-tube vacuum refining device, belongs to molten alloy treatment, and especially relates to external refining of molten steel. The device comprises a dip tube, a ladle and a buggy ladle. The dip tube consists of an upper vacuum chamber and a melting chamber. A top lance, a vacuum pumping mechanism and a charging mechanism are installed on the dip tube. And the bottom of the ladle is provided with a permeable brick. The device is characterized in that the dip tube has a flat bottom, an approximately elliptic cross section, and two long side walls connected with two short side walls straightly and parallelly, also the arc of the ellipse is concentric with the ladle, the long axis of the ellipse is over a diameter line of the ladle, and a temperature measuring and sampling area forms between the long side walls and the inner wall of the ladle, a temperature measuring and sampling mechanism is arranged deeply in the area, the bottom argon blowing permeable brick is disposed eccentrically, and an air vent of the brick is positioned right below the dip tube. With the advantages of novel conception, simplified and compact structure, easy manufacturing, low cost, convenient operation, the device of the invention improves the stability and reliability of refining, enhances the decarbonization speed, and strengthens decarbonization stability. Being convenient for temperature measuring and sampling, the device provided in the invention ensures a smooth production process, and is beneficial to improve steel quality and lower production cost.