The invention provides a radioactivity area determining method based on a virtual point detector principle. The method includes the steps of obtaining a fitting parameter of the distance from a virtual point detector to the lower end face of the detector in a fitting mode, obtaining virtual point source positions corresponding to ground radioactivity areas in a calculating mode, building a function relation between radiuses r of the ground radioactivity areas evenly distributed and the virtual point source positions x corresponding to the ground radioactivity areas, carrying out back calculation of radius parameters of the ground radioactivity areas and the like. By means of the virtual point detector theory and the virtual point source technique, the problem that in the prior art, radioactivity suspicious area boundaries cannot be determined at a high level is successfully solved, and aerial development becomes an effective detection means.