Abstract: This paper provides a new method to determine mixed component system by multiwavelength absorbartce ratio difference spectrophotometry and summarized the basic principle and experimental technique of this method. This method was applied to assay the contents of two kinds of mixed component system of diphenhydramine hydrochloride and ephedrine hydrochloride, amidopyrine and caffeine without separation. The mean recoveries with coefficient of variation (%) was 101.0% (1.0%) for ephedrine hydrochloride; 94.86% (0.56%) for diphenhydramine hydrochloride; 100.3% (0.70%) for amidopyrine and 100.1% (0.75%) for caffeine. The results obtained are satisfactory both in recovery and in precision. It is a simple and rapid method to determine mixed component system.