Abstract: Fly ash can be used as a concrete admixture. If the sulfur content in fly ash is too high, the volume of concrete will change, resulting in expansion cracking and other problems. Therefore, accurate determination of sulfur content in fly ash is of great significance for fly ash application. The sulfur content in fly ash is determined by the barium sulfate mass method, which has a complicated process, complicated operation, and strict requirements for personnel and operation. It was found that the sulfur in fly ash was mainly combined with calcium by SEM-EDS. In order to avoid the influence of the matrix effect, a synthetic calibration sample was prepared by adding calcium sulfate to the matrix of the fly ash sample. The sulfur content in fly ash was determined by the high frequency combustion infrared absorption method. To explore the best experimental scheme, the orthogonal test with 4 factors and 3 levels was designed. The 4 factors were quality of flux (iron, tungsten, tin), and the adding order of flux and sample. The influence of analysis time, sample weight and other parameters on the determination of sulfur content was also investigated. The calibration curve equation was y=2194.4x+8.21, the correlation coefficient was r=0.9998, the detection limit of the method was 0.00036% (mass fraction, the same below), and the limit of quantitation was 0.0012%. The relative standard deviation (RSD) of sulfur content in fly ash was 0.6%−2.1% and the results were consistent with those of the barium sulfate mass method. Compared with the barium sulfate mass method, this method is simple to operate and can be used to rapidly and accurately determine sulfur content in actual fly ash sample.