Abstract: Octadecyltrimethylammonium chloride (OTAC) is the active ingredient of Lüqingling,mainly used as orange miticide.The indirect photometric chromatographic method for determination of OTAC was established.And the chromatography condition was optimized:ODS column,water (with 2.5 mmol/L SAA)=25∶ 75 as mobile phase,flow rate at 0.3 mL/min,column temperature at 38 ℃ and the detection wavelength at 294 nm. The detection limit was 1.968×10-9 g and the linear correlation coefficient was 0.998 9. According to the recommended concentration of 66.7 mg/L,the residual amount was from LOD to 4.157 mg/kg. Thirty days after the application,no residue was detected in the soil.