Abstract: A 220 Rn accumulation box with the size of 350 mm×250 mm×180 mm is made of perspex plate. 220 Rn level in the chamber is monitored by a special scintillation cell. The calibration factor for 220 Rn concentration measurement is determined by the detection efficiency of α particles emitting from radon and its progenies. 220 Rn gas and circulation air are mixed together before entering the accumulation box with a flow rate of about 13 L/min. 220 Rn concentration can be changed by adding different volumes of delay boxes or by controlling gas flow rate in the 220 Rn input circuit. The simple 220 Rn chamber has a good stability of concentration with deviations of 1.0% during 10 h continuous measurement, and a good uniformity with deviations of only 0.3%. The uncertainty of the system is less than 5.6%.