Abstract: The following steps are taken in selecting the groups of three wavelength with microcomputer: First, absorbances of the constituent under test and interfering substance are determined and printed by using UV-250(or UV-300) When UV—751G is used, a proper wavelength range should be chosen. Then, selecting optimal groups of three—wavelength is performed through the program "UV—TWSI" with microcomputer TRS-8O and the 10 groups, with maximal absolute values of △A are printed. Finally, all the calculations are conducted through the program UV-TWC2. Analysis of chlorpromazine hydrochloride injection is taken as an example. According to steps described-above, the manual spectrophotometer ( UV—751G) is used to determine five batches of samples the average recovery is found to be 100.3±0.28%, and the accuracy and precision of the results are found to meet the analytical requirements of the pharmaceutical preparations, the relative standard deviation is in the range of 1%.