Abstract: The optimum experimental condition for simultaneous UVspectrophotometric determination of the contents of aspirin, phenacetin and caffeine in aspirin compound tablets (APC) and the basic principle and application of partial least squares method(PLS) in simultaneous multicomponent determination have been studied. Confidence intervals of the three components are 100.1±0.23%(aspirin), 100.0±0.25% (phenacetin) and 100.1±0.33% (caffeine) (confidence 95%). No information has ever been available in the literature for the application of PLS in pharmaceutical analysis. Compared with other traditional computing methods, PLS is a more perfect multicomponent determination method. It is especially applicable to analyzing samples in batches. It is faster and produces more accurate and reliable results. PLS provides a new method for in-line UV-visible spectrophotometric automation.