Through the analysis of the effects of ancient weights and measures on dosage of classical prescription,the historical changes of dosage conversion of ZHANG Zhongjing's classical prescription were observed.On the basis of respecting the dosage of classical prescription,ancient and contemporary doctors control dosage flexibly combining with clinical practice,which was systematically introduced.In the future,the dosage conversion and application of ZHANG Zhongjing's classical prescription should be explored on the basis of comprehensive consideration of clinical efficacy and safety.The dosage-effect relationship should be clarified,and the optimal clinical dosage should be sought,so as to improve clinical efficacy.