A method for the analysis of acetic acid and propionic acid in the gas by GC-FID was set up. Acetic acid and propionic acid in the gas was adsorbed by silicon tube，desorbed by acetone and then determined by GC. The calibration at the concentration range 50-2000 mg/L was linear by the optimized method. The detection limit was 1.0 mg/m3 and 0.5 mg/m3.The relative standard derivations was 2.2% and 1.9%，and the recovery was higher than 94%.