In order to prevent the existence of natural radon and thorium,which lead to false alarms with the on- line aerosol continuous monitoring instrument at nuclear power plant. A new type of aerosol radioactivity monitoring instrument has been developed. For this type of instrument,three steps are adopted for the measurements,which are sample collection,remain and measurement. Temperature compensation,self- absorption correction are introduced. Besides,two- energy discriminating method and α/β ratio method are adopted to modify the precision,so that the influence of radon and thorium can be avoided. The interference of radon and thorium could be solved by this way.