Abstract: A lot of observational data obtained both at home and abroad suggest that before some strong earthquakes, anomalous variations of radon concentration appear in the chemical composition of groundwater. In order to further study these anomalies and get reliable precursory anomalies, a special packet of precursory data processing has been developed recently. By using a computer program of one-order difference method, it is discovered that the dispersion of radon content systematically increases before earthquakes at some observation posts which have fairly good observation conditions. This change may be a new criterion for evaluating short-term and imminent earthquake precursors.