The scientific understanding of pain has stayed on a traditional classic “pain pathway” for many years. However, with the continuous progress of clinical diagnosis, this pathway obviously cannot explain all pain phenomena, and the understanding and interpretation of pain needs to be updated. Based on the influence of comprehensive medicine based on philosophy and the continuous summary and research of clinical cases of somatic symptoms, this paper takes the biological defense mechanism as the basic starting point, analyzes the position of pain in the biological defense mechanism, and reinterprets and defines pain from the new point of view of comprehensive medicine with the new perspective of biological defense mechanism. In addition, this paper also presents the new treatment principles of pain according to the occurrence mechanism through the perspective of biological defense and psychosomatic mechanism, and puts forward new principles of treatment and the view that “defense” is the fundamental purpose of all biology, aiming to provide a reference for the reinterpretation of more diseases and the research on the nature of biological behavior.