Abstract: It was of great significance to have an effective method for the field analysis of water sample from formation water of oil field for its residual amount of imidazoline corrosion inhibitor (IMCI), in order to control the optimum amount of IMCI to be added to the piping system and to attain at a high efficiency in the anticorrosion of the piping system. UV spectrophotometry was tested for use in this case, but serious interference from the co-existing impurities in the water sample was encountered making the determination impossible. To overcome this obstacle, a scanning stepping of 5 nm was chosen, and then its 2nd-derivate was derived. It was shown that the interference mentioned above was effectively eliminated by this method, and the absorbance of IMCI was detected at the wavelength of 240 nm. Linear relationship between values of 2nd-derivates and the mass concentrations of IMCI was found in the range from 10 to 100 mg·L^-1. Detection limit (3s) found was 0.07 mg·L^-1. Test for recovery was made by standard addition method, giving results in the range of 99.7% to 102%. Values of RSDs (n=6) found were in the range from 0.83% to 6.3%. In the analysis of unknown sample, 1 L of the water sample was taken from the piping according to the amount of IMCI added, and it was centrifuged and filtered through filtering paper, and the filtrate was analyzed by derivative UV spectrophotometry under the working condition of the instrument.