Abstract: In order to satisfy the need of OSI (on-site inspection), the equivalent gas radioactive source was developed, which was used to implement the efficiency calibration for ¹³³Xe 81.0 keV γ-ray. The radioactive source was composed of foam grains as matrix material and ²²⁶Ra as parent nuclide, and encapsulated by stainless steel and carbon fiber window to decrease the attenuation of γ-ray. The experiments on tightness and uniformity were done after the source was manufactured. When the source became equilibrium between ²²⁶Ra and ²¹⁴Pb, the efficiencies of X-ray of ²¹⁴Pb were measured. Then the interpolation calculation with determination values was done to obtain the efficiency of ¹³³Xe 81.0 keV γ-ray. To verify the accuracy of calibration results, the experiment was finished on the same measurement conditions with a ¹⁵⁵Eu source whose matrix and geometry dimensions were the same as ²²⁶Ra source. The result indicates that the relative deviation of detection efficiency between ¹⁵⁵Eu 86.5 keV γ-ray and ²¹⁴Pb 87.3 keV X-ray is less than 1.0%.