To analyze the historical understanding of yin and yang toxin,and to explore the modern clinical application of Shengma Biejia Decoction.Methods：Based on medical history and literature,yin and yang toxin in different historical periods was interpreted and compared.Results：Cold and heat properties of yin and yang toxin recorded in different historical periods are different; Realgar and Pericarpium Zanthoxyli can be used in Shengma Biejia Decoction in the treatment of modern heat hemorrhagic diseases similar to yang toxin.Conclusion：There are differences in the historical understanding of yin and yang toxin,which may be due to the fact that yin and yang toxin did not represent the same disease in the past,or the clinical manifestations of yin and yang toxin were changed because of being affected by different toxic qi.