The utility model relates to a sampling system suitable for sampling of low concentration sulfur trioxide, including first thermostat water bath, second thermostat water bath, ice bath pot, drexel bottle and aspiration pump, be equipped with a condenser coil in the first thermostat water bath, be equipped with the 2nd condenser coil in the second thermostat water bath, be equipped with the hydrogen peroxide solution in the drexel bottle, a condenser coil's no. 1 serves and is connected with sulfur trioxide sampling rifle, is equipped with the absorption bottle in the ice bath pot, is equipped with the isopropyl alcohol in the absorption bottle, and the absorption bottle links to each other with the drexel bottle, and drexel bottle and aspiration pump link to each other, the utility model discloses an advantage: through processings of sampling of sulfur trioxide sampling rifle flue gas, sulfur trioxide condensation and collection are realized through the isopropyl alcohol in looping through and getting into the absorption bottle of ice bath pot with the condensation of second thermostat water bath after first thermostat water bath is handled to the flue gas, are equivalent to present single individual condenser coil condensation, can guarantee the complete condensation of sulfur trioxide in the flue gas, collection, improvement condensation efficiency, can realize the flue gas in sulfur trioxide collect totally.