Abstract: Capitalizing on the unique advantages of phosphate solutions, such as inhibiting selenium valence transformation through competitive adsorption and stabilizing system pH via buffering effects, this study optimized key parameters including the solid-liquid ratio. A method based on phosphate mixed solution extraction coupled with atomic fluorescence spectrometry differential subtraction was established for determining Se(IV) and Se(VI) in soil. Compared with strong acid digestion (which may disrupt valence equilibrium), sequential extraction (which is operationally complex and prone to high errors), and in-situ valence analysis (which requires large-scale instruments, and is costly), this proposed method enables stable extraction of Se(IV) and Se(VI) under mild conditions while effectively avoiding valence transformation interference. The optimal extraction parameters were determined as follows: 0.2 mol/L K₂HPO₄/KH₂PO₄ mixed solution, solid-liquid ratio of 1DK∶KG-2mm10, and shaking at 30 ℃ for 120 minutes.Precision tests yielded relative standard deviations (RSD) of 2.69-12.9% for Se(IV) and 6.43-9.28% for Se(VI), indicating low data dispersion. Spike recovery experiments showed high accuracy, with Se(IV) recoveries ranging from 90.0-100.0% and Se(VI) recoveries from 92.5-110.0%. The procedure is straightforward, does not require large-scale instrumentation, significantly reduces analytical costs, maintains detection quality, and is suitable for the routine and efficient determination of Se(IV) and Se(VI) in soils.