The invention provides an on-line sampling device and method for sulfur trioxide to reduce the human operation error of sampling personnel and improve the sampling accuracy. The device comprises a low-temperature absorber, an isopropanol sampling liquid extraction system and an air extraction system; the air extraction system comprises a gas-liquid separator, a gas mass flow meter and an air pump,wherein the gas mass flow meter and the air pump are successively connected to a gas outlet of the gas-liquid separator; the low-temperature absorber comprises an isopropanol absorption liquid bottle, a flue gas input pipe and a flue gas output pipe; the isopropanol absorption liquid bottle is filled with isopropanol absorption liquid and placed in an environment of -2 to 2 DEG C; the input end of the flue gas input pipe is connected with a heating gun, and the output end is inserted into the absorption liquid; the output end of the flue gas output pipe is higher than a set absorption liquidlevel, and the output end is connected with a gas inlet of the gas-liquid separator; the isopropanol sampling liquid extraction system includes an isopropanol sampling liquid output pipe, a suction peristaltic pump and a sampling collector which are successively connected to the bottom of the isopropanol absorption liquid bottle.