A new method for the measurement of radon is described in this paper. The characteristic that initial RaA carries positive charges was used to make it possible to drift RaA onto the surface of the barrier detector in an oppropiate electric field. The activity of RaA on the detector surface can be measured in high efficiency and the concentration of radon can also be calculated. The new method is distinct from the conventional one using semiconductor device. The Detector acts not only as a signal monitor but also as an ion collection electrode. The sensitivity for one litre measurement chamber is nearly two order higher than that of the conventional instrument with barrier detector for measuring radon.