Octadecylamine is a flotation collector commonly used to produce potassium chloride by cold crystallization-positive flotation, and its dosage will significantly affect the flotation separation efficiency. In addition, octadecylamine will adsorb on the surface of potassium chloride and inevitably remain in potassium chloride products, which is inconducive to developing high-purity potassium salt products. To meet the requirement of determination of octadecamide potency in potassium chloride, an extraction spectrophotometry was developed, based on the principle of van der Waals force and hydrogen bonding between octadecylamine and bromophenol blue with butyl acetate as extractant and bromophenol blue sodium salt as chromogenic agent. The effects of solution pH, dodecyl morpholine, co-existing salt, equilibrium time, and the amount of chromogenic agent on the concentration measurement of octachylamine were investigated. The results show that when the solution pH increases from 3 to 9, the absorbance of the extract decreases because the complexation between octachylamine and bromophenol blue weakens. The polarity of the complex molecules increases. When the solution pH is less than 5, dodecyl morpholine and bromophenol blue can also form colored complexes. While pH is between 6 and 9, the absorbance tends to zero. The ionic strength of the coexisting solutions of potassium chloride, sodium chloride, potassium sulfate and magnesium chloride increases, weakening the hydrogen bonding. Still, the salting-out effect of the complex molecules is enhanced, and the trace Li+, NH+4 and B in the solution have little effect on the absorbance. The absorbance does not conform to Lambert-Beer law when the amount of chromogenic agent is too large. Besides, equilibrium time has little effect on the absorbance. The determination conditions in potassium chloride solution are as follows: The solution pH is 6, the ionic strength is 1 mol·L-1, and the amount of 2 mmol·L-1 chromogenic agent is 0.5 mL. 5 mL butyl acetate is added to the 25 mL aqueous solution adjusted by buffer after 5 min of reaction for extraction. After 2 min of delamination equilibrium, the absorbance of the extraction solution is tested at 458 nm. The working curve is A=0.049 49c+0.066 24 (R2=0.992 3, ε=1.33×104 L·mol-1·cm-1, 0~10 mg·L-1). The relative standard deviation of this method was 0.33%~2.63%, the mean relative error was -0.90%, and the mean relative error of the system of octadecylamine and dodecylmorpholine was -0.25%. The content of octadecamide in the filter liquor derived from washing potassium chloride after positive flotation was 8.66 mg·L-1 and recoveres were 95.5%~106% by this method. The extraction spectrophotometry has been proven to be appropriate for detecting the concentration of octadecamide in the production of potassic fertilizer in salt lakes.