The changes of the content of psoralen and isopsoralen in the formula for tonifying kidney-yang were determined by orthogonal test,L₁₂(2¹¹),and thin-layer chromatography. The results showed that the extracted elements of Fructus Psoraleae(FP)was markedly increased by pounding before decocted.In case the FP was decocted together with the Fructus ujubae and Radix Astragali(drugs for strengthening the spleen),the extracted elements of FP are also increased markedly. The experiment also showed that the extracted elements of FP are markedly increased when decocted together with Radix Paeoniae Alba, Radix Salviae Miltiorrhizae,Radix Angelicae Sinensis and Semen Cucustae, but a tendency of decrease by decocted with Radix Polygoni Multiflori and Radix Rehmanniae Preparata. These findings revealed the inherent basis ofthe principle of compatibility is“Mutual promoting”.