Abstract: In this paper, a method for the determination of the contents of sulfur trioxide in gypsum mines by high frequency infrared carbon sulfur analyzer with satisfactory results was established. The detection limit of the method was 0.003 0%. The proposed method was applied to the analysis of certified reference materials of gypsum (GBW03109a, GBW03110), and obtained results in consistent with the certified values, the RSD(n=9) was 0.32%~0.81%. By the analysis of the certified reference material of gypsum (GBW03111), there was no significant difference between this method and the routine gravimetric method of BaSO₄. Recovery rates obtained by standard spike method were in the range of 96.4%~104.0%.