The function, circulation, distribution and characteristics of Weiqi (Defensive qi) in ancient literatures are retrieved and extracted in this article to analyze and study the theory of "applying acupuncture according to the properties of Weiqi (Defensive qi)" in Internal Classic. The result shows that Deqi (arrival of qi) is closely related with Weiqi (Defensive qi). The circulation of Weiqi (Defensive qi) has its own temporal regularity, which should be followed by the application of acupuncture. The gathering and scattering of Weiqi (Defensive qi) can be used to explain the relationship between distribution of acupoint and curative effect. Circulation of Weiqi (Defensive qi) is influenced by yang qi in the universe which varies according to seasonal factors. Therefore, acupoint selection according to time should not be adopted mechanically. Furthermore, the circulation of Weiqi (Defensive qi) can be either independent or identical with the channels. Therefore, it is still under discussion whether the needling sensation is actually transmit along the channels in the experiment of "propagated sensation along channels".