Abstract: This paper reveals a general principle of multiwavelength spectrophotometry and puts forward a multiwavelength coefficient method to determine mixed components system. The determination of the coefficients and the connection between the coefficients with the method are discussed in the paper. Using several groups of coefficients,according to the method, a mixed components of chlorpromazine hydrochloride and promazine hydrochloride were assayed at the same time. The laboratory results of recoveries indicates that the precision and the accuracy are good, and, different coefficients do not make a notable impact on the method.