OBJECTIVE： To investigate the decoction amount changes of verbascoside， gallic acid in single decoction and combined decoction of rehmanniae radix preparata， corni fructus and poria， and provide scientific basis for further study of effective substance basis. METHODS： The single decoction， combined decoction with each 2 medicine and combined decoction with the 3 medicines of rehmanniae radix preparata， corni fructus and poria， were respectively prepared. HPLC was adopted to detect and compare the decoction amount of active component verbascoside in rehmanniae radix preparata and active component gallic acid in corni fructus in each group. RESULTS： Compared with single decoction of rehmanniae radix preparata， the decoction amount of verbascoside was decreased in combined decoction of poria+rehmanniae radix preparata （P＜0.01）， decoction amount of verbascoside was increased in combined decoction of corni fructus+rehmanniae radix preparata or combined decoction with the 3 medicines （P＜0.01）. Compared with single decoction of corni fructus， the decoction amount of gallic acid in each combined decoction was decreased （P＜0.01）. CONCLUSIONS： The combined decoction of rehmanniae radix preparata， corni fructus and poria has certain promotion and inhibition effects on the decoction of verbascoside in rehmanniae radix preparata and inhibition effect on the decoction of gallic acid in corni fructus. It is speculated verbascoside may be one of the main components in combined decoction playing the role of effectiveness.