In the present paper, the author sums up and analyzes descriptions about needle-insertion depth in Chinese classical medical book Huang Di Nei Jing (The Yellow Emperors Internal Classic). In many chapters of Nei Jing, the needle-insertion depth is stressed to be various according to 1) the deficiency or excess of syndromes, 2) the patients constitution, 3) the severity of disease, 4) the duration of disease, 5) the location of disease, 6) the patients age, 7) the location of the needled acupoint, 8) the season, 9) the patients temperament, 10) the pulse condition, 11) the state of "Deqi", and 12) the location of the running course of meridians. In addition, different kinds of diseases and different stages of diseases also need different depths of needle insertion, different manipulating skills and different stimulating quantity.