Abstract: The 0.250 0 g of geological sample was taken, and about 1.5 g of sodium peroxide was added. The mixture was melt at 700 ℃ for 20 min, and cooled down. Then 30 mL of slightly boiling water was added to leach the melt, and 5 drops of anhydrous ethanol were added. The solution was made its volume up to 50 mL by water. The 10.00 mL of supernatant was taken, and 3-4 drops of 1 g·L⁻¹ phenolphthalein indicator were added. The 10% (volume fraction, the same below) hydrochloric acid solution was added to neutralize until colorless, and 10 mL of 10% hydrochloric acid solution and 5 mL of the mixed solution containing thiourea and ascorbic acid with the same mass concentration of 50 g·L⁻¹ were added. The solution was diluted to 50 mL by water for determination of Sn by atomic fluorescence spectrometer at lamp current of 80 mA, atomizer height of 8 mm, photomultiplier negative high voltage of 300 V, carrier gas flow of 400 mL·min⁻¹, shielding gas flow of 1 000 mL·min⁻¹, and potassium borohydride mass concentration of 30.00 g·L⁻¹. It was shown that linear relationship between the mass concentration and fluorescence intensity of Sn was kept within 150.00 µg·L⁻¹, with correlation coefficient of 0.999 3, and detection limit (3s/k) of 0.42 µg·g⁻¹. The proposed method was used for the analysis of geological reference materials, giving RSD (n=12) of the determined values of 3.7%, absolute values of the relative error less than 10%, and absolute values of logarithmic difference (ΔlgC) between determined values and certified values less than 0.10. The proposed method was used for the analysis of soil, water sediment, and rock samples, and the determined values were basically consistent with those obtained by arc emission spectroscopy.