Abstract: A method was proposed for the simultaneous determination of 4 key impurities, including guanidine nitrate, nitroguanidine, 2-nitroaminoimidazoline and 2-chloro-5-chloromethylpyridine in imidacloprid insecticide by high performance liquid chromatography (HPLC). The sample of 2.000 g was dissolved in 20% (φ) methanol solution, and then made its volume up to 100 mL. The Agilent Eclipse XDB C₁₈ chromatographic column (250 mm×4.6 mm, 5 μm) was used as stationary phase, and a mixture of 20 mmol·L^-1 ammonium acetate solution with 10 mmol·L^-1 heptafluorobutyric acid and methanol in different volume ratios was used as the mobile phase for gradient elution. External standard method was used for quantification. As shown by the results, linear relationships between values of peak area and mass concentration of 4 key impurities were found in the same range of 0.4-40 mg·L^-1, with detection limits (3S/N) ranging from 0.13 to 0.15 mg·L^-1. Tests for recovery were made by standard addition method, giving results in the range of 95.4%-103%, and RSDs (n=6) of the determined values all less than 2.0%.