The prescription of Sini-Decoction has changed in the past dynasties，and the different medical practitioners have various views about the monarch drug in the prescription. Based on the analysis from TCM theory，the medical commonness of Sini-Decoction types in “Febrile Disease”，the amount of drugs in the prescription，the indispensible drug in the prescription and the pharmacological researches and so on，it is concluded that the monarch is uncertain and depends on the basis of symptoms or causes，the greater or less urgency.