Objective To study the inhibitory effect of Huidu Yinhua powder from the Orthodox Manual of External Medicine on methicillin-resistant Staphylococcus aureus(MRSA),virulence factor α-hemolysin(Hla)activity,and biofilm formation,and to explore the optimal ratios of Huidu Yinhua powder and provide experimental support for its use.Methods The inhibitory effects of Huidu Yinhua powder and the herbs in the formula on USA300 were analyzed by the minimum inhibitory concentration(MIC),minimum bactericidal concentration(MBC),and disk diffusion assay(K-B method).Hemolysis,neutralization,oligomerization,and Western blot assays were used to verify in which form the drug inhibits the activity of virulence factor α-hemolysin(Hla).A biofilm assay was performed to evaluate the inhibitory effect of Huidu Yinhua powder on biofilm.Orthogonal experiments were performed to explore the optimal ratio of Huidu Yinhua powder.Results Huidu Yinhua powder inhibited the MRSA strain with a MIC90 of 64 mg/mL and an MBC of 256 mg/mL with antibacterial circle diameter of(7.50±0.50)mm.Huidu Yinhua powder inhibited Hla activity by inhibiting Hla secretion.The minimum effective concentration(MEC)was 16 mg/mL,and the MEC of biofilm was 8 mg/mL.In Huidu Yinhua powder,honeysuckle and astragalus only affected the hemolytic activity of MRSA and biofilm formation without inhibiting bacterial growth.The hemolytic activity and biofilm of MEC were both 32 mg/mL.Glycyrrhiza had a strong bacterial inhibitory capacity with a MIC90 of 8 mg/mL and biofilm MEC of 1 mg/mL without showing inhibitory hemolytic activity at subinhibitory concentrations.The orthogonal experiment showed that,at a ratio of honeysuckle,astragalus,and glycyrrhiza in Huidu Yinhua powder of 1∶2∶4,the MIC90 was 16 mg/mL,MEC of hemolytic activity was 8 mg/mL and that of biofilm was 4 mg/mL,both of which were the lowest among the nine groups.Conclusions Huidu Yinhua powder affects the hemolytic activity and biofilm formation of MRSA at subinhibitory concentrations with the optimal ratio of honeysuckle,astragalus,and glycyrrhiza being 1∶2∶4.