A new flow injection analysis（FIA） system with a flat pH electrode was established for the rapid determination of acetic acid in vinegar.The FIA system was optimized by alteration of injection volume，flow rate，carrier concentration and length of reaction tubes.With sodium hydroxide solution as carrier stream，the peak area obtained from the lab assembled FIA system were proportional to the logarithm concentration of acetic acid in the range of 4639×10-4-0212 mol?L-1.The method was applied in the detection of acetic acid in four vinegar samples，and the results obtained were in good agreement with those obtained by the standard manual titration method.The sensitivity of this method was ten times higher than that of the conventional titration method，and the analysis rate was up to 60 samples per hour.By changing the concentration of carrier stream（sodium hydroxide），the acetic acids at different concentration levels can be titrated without any other adjustments to the FIA system.