Abstract: After drying, crushing, mixing and sieving, 0.1 g of the raw grain sample was put into a hard glass test tube. Mixed solution (3 mL) composed of perchloric acid and nitric acid at volume ratio of 1∶4 and 1 mL of 80 g·L^-1 calcium chloride solution (to inhibit the volatilization of selenium element) were added, and the mixture was placed overnight, and digested in a tubular furnace under program temperature conditions in the next day. After cooling to room temperature, 1 mL of 6 mol·L^-1 hydrochloric acid solution was added, and the mixture was heated in the tubular furnace at 100 ℃ for 10 min, to reduce the hexavalent selenium in the digestion solution to tetravalent selenium. After cooling to room temperature, 1 mL of hydrochloric acid was added, and the mixture was diluted to 10 mL with water. The solution obtained was introduced into the atomic fluorescence spectrometer, and detected according to the following optimized instrument working conditions: the lamp current was 80 mA, the negative high voltage was 300 V, the mass concentration of sodium borohydride in the reducing agent was 20 g·L^-1, and the volume fraction of current carrying hydrochloric acid solution was 10%. It was shown that the mass concentrations of selenium element were linearly related with corresponding fluorescence intensities within 20.0 μg·L^-1, with detection limit (3s/k) of 0.024 1 μg·L^-1. The proposed method was applied to the analysis of reference materials of total selenium in rice (GSB-1), in wheat (GSB-2) and in maize (GSB-3). Compared with the first method in the national standard of GB 5009.93-2017, the determined values were closer to the certified values, and RSDs (n=5) of the determined values were lower.