The invention relates to gaseous radioactive waste processing technology, specifically to a testing system for the performance of a drying agent of a dehumidifying and drying unit of a gaseous radioactive waste processing system. The testing system comprises a nitrogen supplying device and a compressed air supplying device; the nitrogen supplying device and the compressed air supplying device are separately connected with a first measuring tank; the outlet of the first measuring tank is connected with the inlet of a drying agent testing bed; the outlet of the drying agent testing bed is connected with a second measuring tank; the outlet of the second measuring tank is connected with a chimney; pressure sensors and temperature sensors are arranged on the drying agent testing bed; and a pressure sensor and a humiture sensor are respectively arranged on the first measuring tank and the second measuring tank. The testing system can adjust relative humidity and temperature of air flow so as to realize testing of the performance of the drying agent under different parameter conditions.