The mechanism and causes of “Wei qi internal invading” are analyzed from the ascending and descending, exiting and entering of Wei qi. According to the physiological and pathological characteristics of Wei qi, it is concluded that the ascending and descending, exiting and entering channel of Wei qi movement is the “lower jiao- middle jiao- upper jiao- lung governing diffusion- muscle- body surface”. This channel dysfunction can lead to the “Wei qi internal invading”, with the clinical symptoms of fever, fullness and skin insensitivity. The treatment is mainly to dredge the channel of qi movement. One proven case is presented.