Ulcerative colitis (UC), a kind of chronic nonspecific intestinal inflammation with unknown etiology, is very difficult to cure. It recurs often and even is accompanied with parenteral lesion. It has been rated as one of contemporary refractory diseases by World Health Organization. The author combines the pathological mechanisms of UC with the poison-evil theory and the collateral disease theory, and puts forward that "toxin impairing intestine collateral" pathogenesis theory. We believe that longer accumulation of damp-heat stasis toxin that damages the intestine collateral is the key cause for recurrence and relapses of UC. We explained its theory foundation in terms of pathogenesis of UC from Chinese medicine and modern medicine. The "toxin impairing intestine collateral" theory might perfect the syndrome typing system for UC, and provide a new way of thinking for treating UC.