To analyze and identify the constituents in rat plasma after oral administration of the active   fraction of Corydalis yanhusuo, a LC-MS/MS method was established.  The constituents absorbed into blood, their original crude drugs and their metabolites were identified either by comparing the retention time and mass spectrometry data with that of reference compounds or by mass spectrometry analysis and retrieving the     reference literatures.  Nine species are the original form in Corydalis yanhusuo, moreover, some metabolites in blood identified as glucuronide were found.  The constituents absorbed into blood and the possible metabolites which demonstrate to originate from the active fraction of Corydalis yanhusuo are responsible for the observed efficacy.  Its serum pharmacochemistry should be subjected to complete investigation so as to illuminate the pharmacology and active mechanism of the active fraction of Corydalis yanhusuo.