Collateral disease theory is an important pathological doctrine of Chinese Medicine. This paper explored Chinese medicine pathology of chronic atrophic gastritis（ CAG） based on collateral disease theory. It raised that "spleen-deficiency

collateral-stagnation

toxin-injury " is the basic pathogenesis of CAG

and "spleendeficiency"

"collateral-stagnation"

"toxin-injury"are the fundamental factors of pathogenesis developing of CAG

mostly manifesting as the developing rule of spleen and stomach deficiency to gastric collateral stagnation

and then to toxin harming collateral. Among those

spleen and stomach deficiency is the initial factor and morbidity root; gastric collateral stagnation is the key condition of CAG causing disease; toxin harming collateral is the important factor of CAG developing into precancerous lesions. The manifestation of this disease relatively fits the feature of collateral diseases. Through collateral disease theory

the pathological feature of CAG can be better clarified

and the pathological developing rule of CAG can be understood more deeply.