Abstract: In this work, a gas scintillation proportional counter (GSPC) with effective detection area of 300 cm² was built. The wavelength shifting material was used to couple the photomultiplier and scintillator to achieve the light collection. The performance testing was done to test the detector. The result shows that the energy resolution for ¹⁰⁹Cd 22.6 keV X ray is 7.5%, and the total detection efficiency is 12.4% when the source-detector separation distance is 3.2 cm. The detector studied in this work could be used to measure the low energy X and γ radioactive sample in radiation contaminated facilities, and to measure the internal intake activity of transuranium elements.