Abstract: Stem bromelain was a typical sulfhydryl protease, featured as poor stability and easy of inactivation. Inactivation and activation mechanisms to stem bromelain were examined in this paper, by studying the effects of inhibitors and activators on stem bromelain activity, kinetic parameters and spectral characteristics, and summarizing rule stem bromelain structural activity variation.Results showed that the CD spectra of stem bromelain effected by inhibitors and activators appears as decreasing α-Helix, and increasing β-Sheet, β-Tum and Random in varied degree.The inhibitors impacted on molecular conformation more than activators do.Such impact caused also UV absorption spectra varying.Activator impacted a secondary- structure reaction mainly below 230nm, while alcotate and inhibitors additionally caused exposure of Trp, Tyr and Phe, which meant a significant enzyme molecule conformation react. This study provided a theoretical basis for the consequent research of enzymatic modification.