Abstract: A new source term inversion method was presented to accurately estimate leak rate of airborne radionuclides released into the environment under the circumstance of nuclear power plant severe accidents. The method is based on containment short-range gamma-ray spectrum data obtained by mobile detection points arranged near the containment. The choice of appropriate detection range, effective arrangement of mobile detection points, and accurate estimate of leak rate for a single radionuclide were studied. As a result, ⁸⁸Kr is selected as the characteristic nuclide among all airborne radionuclides due to its relatively high first characteristic gamma-ray energy and high leak rate. The detection range is determined as within 100 m from the containment. The relation between gamma-ray spectrum data and leak rate of characteristic radionuclide is established to obtain inversion result numerically.