Abstract: In order to overcome the defect of the pretreatment process of acidic organic pesticides for soil samples, such as complicated, low recovery and large secondary pollution and so on, in situ derivative method for acidic organic pesticide residues in soil was established. The soil sample, Na₄-EDTA and pure water were mixed sufficiently, and then transfered the sample into a derivative bottle after the water evaporated, used 2,3,4,5,6-pentafluoro-benzyl bromide as derivative reagents for in situ ultrasonic derivatization. The proposed compounds were separated and analyzed by gas chromatography-negative chemical ionization mass spectrometry(NCI) after the extract was purified. A series of factors affected sample analysis, such as complexation extraction condition, derivatization condition, purification condition, negative chemical ionization mass spectrometry condition and soil TOC, were all well studied and carefully optimized. The results show that the linearity range is 5-500 μg/L with the correlation coefficients greater than 0.998 0. The spiked recoveries of different concentrations range from 80% to 110% with the relative standard deviations of 8.2%-13% (n=7). Real sample is analyzed by this method, and some of proposed acidic organic pesticides are detected in some soils.