The invention relates to a fiber drawing furnace mouth sealing device and a control method thereof. The fiber drawing furnace mouth sealing device comprises an extensible sealing circular ring, a connecting piece, an air exhausting and charging device, a laser diameter measuring instrument, an air pressure sensor, and a controller in signal connection with the laser diameter measuring instrument, the air pressure sensor and the air exhausting and charging device and analyzing preform diameter data to control the air exhausting and charging device to perform corresponding air exhausting and charging actions, wherein the laser diameter measuring instrument measures an optical fiber preform diameter d and transmits the diameter to the controller through a data line. The air pressure sensor measures the air pressure p in the extensible sealing circular ring and transmits the air pressure to the controller, and the controller conducts contrast control on air exhausting and charging. The fiber drawing furnace mouth sealing device has the advantages that the size of the extensible sealing circular ring is timely adjusted according to the change of the preform diameter, an optical fiber drawing furnace mouth upper sealing effect is ensured in the whole fiber drawing process, accordingly high-speed stable fiber drawing of an irregular preform is achieved, and raw material cost waste caused by untimely adjustment can be greatly saved due to automatic control.