There is an old saying that moxibustion,leading to sore,is effective.This paper investigated the source of post-moxibustion sore,principal of medical science and the bad disease to testify the old saying.Post-moxibustion sore was firstly recorded in Wuwei Medical Bamboo Slips of Han Dynasty and Synopsis of Golden Chamber written in Han Dynasty.The original meaning was ulceration from burning by post-moxibustion.Post-moxibustion sores have been perceived as the sign of the moxibustion of resolving pus since A-B Classic of Acupuncture and Moxibustion and has the records of purulent infection on moxibustion sore resulted by hot straw sandals in Jin dynasty.With syndrome differentiation and acupoints selection principles,the moxibustion of resolving pus can be effective on the basis of sufficient qi and blood,correct acupoints selection,enough quantitative moxibustion.Otherwise it may result in bad symptoms,if it is with the excessive moxibustion,incorrect acupoints selection,and improper nursing.The safety and efficacy of post-moxibustion sore has been questioned by the later medical master in every dynasty.It was suggested that 8 kinds of wood fire were forbidden.Acupoints that were forbidden to do acupuncture,were set to prevent the harmfulness.It was proved by clinical practice that moxibustion sensation of “Arrival of Qi” has proved the sign of moxibustion effects.Therefore,the old saying that moxibustion must lead to sore,meant that acupuncture quantity were strengthened in order to meet the standard of qi arrival,but not the result of excessive moxibustion.It was not the sign of effective moxibustion therapy.