The principle is "treating the inside from the outside". Acupuncture needles stimulate acupoints, and through the conduction of meridians, regulate the functions of zang - fu organs, ying - wei qi and blood, mobilize the human body's own anti - disease ability, so as to achieve the purpose of preventing and treating diseases. The functions of acupuncture are to regulate yin - yang, strengthen the healthy qi and expel pathogenic factors, and dredge meridians.

1. Regulating yin - yang: When the yin - yang of the human body is in a relatively balanced state, various tissues, organs and zang - fu organs can maintain normal physiological functions. If the yin - yang of the human body loses its balance, diseases will occur. Acupuncture can regulate yin - yang through meridians, acupoint compatibility and manipulation techniques. For example, toothache caused by exuberant stomach fire belongs to excessive yang - heat. It should be treated by clearing and purging stomach fire. Neiting acupoint on the Stomach Meridian of Foot - Yangming is selected, and the reducing method is used for needling to clear stomach heat.

2. Strengthening the healthy qi and expelling pathogenic factors: "Zheng" represents the regulatory and defensive functions of the body, and "Xie" represents all pathogenic factors that cause diseases. Acupuncture can strengthen the healthy qi and expel pathogenic factors. Acupoints with a tonifying effect, such as Guanyuan, Qihai, Mingmen and Shenshu, are mostly used when strengthening the healthy qi. Acupoints with a purging effect, such as Quze and Weizhong, are mostly used when expelling pathogenic factors.

3. Dredging meridians: Meridians are the channels through which the five zang - organs and six fu - organs are interconnected with the skin, muscles, limbs and nine orifices of the body surface. They have the functions of transporting qi and blood, communicating the body, connecting the exterior and interior, up and down, and regulating the functions of zang - fu organs and tissues. When the functions of qi and blood in meridians are disordered, which destroys the normal physiological functions of the human body, various pathological changes will occur. Acupuncture can act on acupoints and meridians, regulate yin - yang, supplement deficiency and purge excess, strengthen the healthy qi and expel pathogenic factors, dredge stasis, regulate qi and blood through the effect of meridian qi, so as to cure diseases and resist pathogenic factors.