Abstract: OBJECTIVE To establish a GC method for determination of acetaldehyde in acetic acid used as pharmaceutic adjuvant. METHODS Acetic acid was diluted by water with the same volume, then neutralized by equivalent sodium hydroxide, sealed immediately and determinated by headspace GC. HP-INNOWAX capillary column (30 m×0.32 mm, 0.5 μm) was used as analytical column, FID detector was used. The column temperature rose by program: the initial temperature was 35 ℃, maintained for 5 min, raised to 120 ℃ with a rate of 30 ℃·min^-1, maintained for 2 min. The flow rate of N₂ was 3.0 mL·min^-1. The headspace injection was used, oven temperature was 80 ℃ maintained for 30 min. RESULTS The influence brought with exothermic reaction was avoided by the dilution of acetic acid, and the salt in test solution generated by neutralization reaction increased detection sensitivity greatly. The calibration curves were linear in the rang of 10.73-107.3 mg·mL^-1(r=0.999 8), the average recovery of acetaldehyde was 95.8%(n=9, RSD=5.0%). Limit of detection of this method was 2.5 μg. CONCLUSION The determination of trace acetaldehyde in acetic acid is solved by simple method of headspace GC, it establish the foundation for the revision of national drug standards.