Abstract： Objective To screen effective inhibitors of 5 α-reductase in vitro,based on in vitro micro reactions.Methods The crude enzyme of 5 α-reductase was obtained from the liver of SD rats,and then established a micro reaction system in vitro.The inhibitory effect of the tested sterol compounds on 5 α-reductase was evaluated by the changes in testosterone levels before and after the reaction.Results The addition of dimethyl sulfoxide(DMSO)to the reaction system solved the problem of low solubility of finasteride and tested sterol compounds in the aqueous phase,ensuring effective trace reactions and obtaining reliable screening results.Conclusion Lanosterol has a good inhibitory effect on 5 α-reductase in vitro,and may become a new type of 5 α-reductase inhibitor.