A rapid new method for the determination of benzalkonium chloride (BAC) in eye drops complex system was developed with ultraviolet spectrum based on oblique projection (OP) and space angle criterion (SAC),in which the content of BAC is based on content of dodecyl dimethyl benzyl ammonium chloride (1227) .By treatment with oblique projection,the pure spectrum of dodecyl dimethyl benzyl ammonium chloride was obtained.The standard spectral matrix was set up by the least squares fitting the spectrum with the corresponding concentration.And then the background spectral matrix was obtained by using high performance liquid chromatography-ultraviolet spectrum(HPLC-UV).After that the contents of 1227 in different kinds of eye drops were calculated by using space angle criterion,and the content of BAC was obtained.The results indicated that the method exhibited a good linearity (r＞0.999 6) for BAC in the range of 1-20 μg/mL.The determination results of this method is close to that of HPLC method,and the relative error is not more than 2.3%.The recoveries were obtained between 99.6% and 101.8%,with RSDs not more than 2.8%.The content of BAC could be directly obtained with multiple wavelength ultraviolet spectrum without complicated sample preparation,and the method has the advantages of short quantitative time,high analysis efficiency,good robustness,and could provide a feasible scheme for the detection of analyte of interest in other complex system.