Abstract：This paper discusses the pathogenesis of cancer from the viewpoint of a sentence in Huangdi Neijing—“Only when yang is compact can it consolidate yin essence”， which reveals the core of yin- yang theory is the compactness of yang. It is found that the pathogenesis of cancer is yang being not sound and yin being not consolidated. In other words，yang cannot prevail yin and yang fails to transform into qi， resulting in yin over- constituting form. Therefore， the treatment method for cancer is to reinforce yang and inhibit yin. Yang qi is generated in the spleen，stored in the kidney，and governed by the heart. And the key to reinforcing yang is to fortify the spleen and stomach so as to supplement and boost the middle qi. It is believed that qi- supplementing method can be used to supplement yang. In the application of this method， Radix et Rhizoma Ginseng， Radix et Rhizoma Glycyrrhizae Praeparata cum Melle and Radix Astragali are commonly used， and Rhizoma Zingiberis Recens，Radix Aconiti Lateralis Praeparata and Ramulus Cinnamomi should be used with caution. The only way for Chinese medicine to develop is to use the highlights of classical theories to guide the clinical practice.