A convenient and fast method for determining available potassium in tobacco planting soil with flame atomic absorption spectrometry was developed, in which available potassium was extracted from soil samples with 1 mol/L ammonium acetate. The results showed that: 1) The effect of soil granularity on extracting efficiency was negligible, the optimal extracting conditions were: 1 mol/L ammonium acetate solution as extracting solvent, the rate of soil weight to extracting solvent volume 1:20, shaking for 120 minutes. 2) Ammonium acetate solution could enhance the atomic absorption degree of potassium, ammonium acetate of 1 mol/L was prepared as a standard mother solution, then a series of standard solution was prepared by the stepwise dilution of the standard mother solution. 3) The limit of detection of the method was 0.018 g/kg with the relative standard deviation of 3.83 % (C=0.7 mg/L, n=10). The analytical standards of soil available nutrients determined by this method were within the error range of reference value. The method was suitable to the determination of batches of tobacco planting soil samples.