OBJECTIVE To modify and optimize gas chromatographic conditions, qualitative and quantitative method on the base of the actual national standard detection method for drinking water on testing iodide with gas chromatography, and detect iodide in drinking water of Shanghai. METHODS Iodobutane derivative was identified by gas chromatography/mass spectrometry, then two capillary columns coupled to electron capture detector were made by qualitative and quantitative analysis. Finally, 100 ml water samples were collected respectively from raw water and finished water of three plants in which water sources were Yangtse river, Huangpu river, and inner river respectively and detected by this developed method. RESULTS The results of mass spectrometry showed that iodide would form iodobutane which could generate 1-iodo-2-butanone and 3-iodo-2-butanone isomers by derivatization. The data of qualitative analysis by two capillary columns revealed that iodobutane was separated completely and the total time of chromatogram separation was 19.33 min.3-iodo-2-butanone with the high response value was selected to quantitatively analyse. The linear range was 1 - 100 µg/L, and the coefficient of determination (r(2)) was 0.9997. The limit of detection was 13 ng/L. Recoveries were between 97.68% and 104.37%, and relative standard deviations were between 2.14% and 4.41%. The results of iodide detected by this method in raw water and finished water in three plants in Shanghai showed that the ranking of iodide's concentration in raw water was Huangpu river (15.14 µg/L) > inner river (6.97 µg/L) > Yangtse river (3.55 µg/L). The level of iodide in finished water of plant 1 (3.55 µg/L and 5.92 µg/L for raw and finished water respectively) and 3 (6.97 µg/L and 9.62 µg/L for raw and finished water respectively) increased slightly except for plant 2 (15.14 µg/L and 1.81 µg/L for raw and finished water respectively). The relative standard deviations of all water samples measured in duplicate were less than 9.73%. CONCLUSIONS The improved method possesses higher degree of sensitivity and accuracy of qualitative and quantitative analysis than actual national standard detection method, and it is fit for trace analysis of iodide in water.