In modern life, chronic tension at work and school, incompatibility of interpersonal relationships at work and at home, and major misfortunes in life are three important factors that cause cancer. Mental factors are closely related to the immune function of human body. The human immune system is regulated by both neurological and endocrine control, which can be considered as follows: Stimulation is effected by human emotions affecting the limbic system of brain, vegetative nervous system, endocrine system and internal organs. Negative emotions, such as mental depression, act on the central nervous system, causing dysregulation of the phytonotropic and endocrine functions, which suppresses the immune function of the organism. As the smoothness among the organism is broken, the cells lose their normal state and function and mutate continuously, giving rise to cancer cells. On the other hand, reducing the production of antibodies in the body hinders the recognition and elimination of cancer cells by lymphocytes, so that cancer cells break through the defense of immune system, proliferate excessively and grow without limitation, forming cancer tumors. Mental factors play a very important role in the occurrence, development and spread of cancer. This has been confirmed by animal experiments. Animals were stimulated with sound and light to produce tension and anxiety, as a result, the defense ability of their immune system was greatly weakened and cancerous tumors previously latent in the chest were induced. In another experiment, tumor cells planted in the buttocks of rats subjected to the same stimulation quickly spread to the lungs and intestines. The reason is that it is these bad mental factors that play the role of “waking up” the sleeping cancer cells, so that they can grow “crazy” and devour the organism with impunity. Therefore, bad emotion is the activator of cancer cells.