Objective: To clarify the interaction mechanism of astragaloside IV and cycloastragenol with bovine serum albumin(BSA) by fluorescence spectroscopy. Method: Scanning the fluorescence quenching and synchronous spectrum through adding astragaloside IV and cycloastragenol to the solution of BSA. Result: The fluorescence quenching mechanism of astragaloside IV and cycloastragenol was static quenching. Under 293 K

the binding constants (KA) of astragaloside IV and cycloastragenol were 1.35×104 and 8.51×104L·mol-1

respectively. And the numbers of binding sites were 0.726 4 and 0.731 2

respectively. Under 310 K

the binding constants of astragaloside IV and cycloastragenol were both decreased. The thermodynamic parameters of astragaloside IV and cycloastragenol with BSA were ΔH<0

ΔS>0

and ΔG<0

the interaction of astragaloside IV and cycloastragenol with BSA were driven mainly by electrostatic interaction. The results of synchronous spectrum showed that both the astragaloside IV and cycloastragenol affected the space conformation of tryptophane. But the space conformation of tyrosine was not affected by astragaloside IV and cycloastragenol. Conclusion: The interaction mechanism of astragaloside IV and cycloastragenol with BSA was clarified by fluorescence spectroscopy. It can supply theoretical supports for the rational administration of astragaloside IV and cycloastragenol.