The invention provides a method for detecting the content of fructo-oligose in liquid milk. The method comprises the steps of adopting fructo-oligose the same as fructo-oligose added to a sample in brand number as a standard substance, and compounding standard working solutions different in concentration; weighing two samples identical in mass, adding the standard substance to one of the samples, adding water for dissolution, and compounding the mixture into a solution with the standard substance added; adding water directly to the other sample, compounding the mixture into a sample solution with the same volume, pre-treating the solution with the standard substance added and the sample solution, and sequentially adding the solution with the standard substance added and the sample solution together with the working solutions into an ion chromatograph for gradient elution, so that chromatograms are obtained separately; comparing the chromatogram of the solution with the standard substance added with the chromatogram of the sample solution, screening out a specific peak used for quantitativeness, and conducting quantitative calculation according to the specific peak, so that the content of the fructo-oligose in the liquid milk is obtained. The method is simple, convenient and fast, no complex operation is needed, errors caused by uncertain factors are decreased to the greatest degree, and result accuracy is remarkably improved.