Through the “Collateral Disease” understanding and analysis， and the TCM for gastric cancer comprehend， deem that gastric cancer belong to collateral disease. Both are because the positive imaginary， qi stagnation， phlegm caused collaterals stasis barrier， stasis barrier existed for a long time， it can be changed into poison to damage collaterals cause disease. Therefore， based on collateral disease theory to discuss the gastric cancer therapeutic， description of gastric cancer treating principles should be dredging collaterals.