The pathogenesis of diabetes mellitus complicated with oral ulcer is interconnected with the Yin-fire theory by LI Dongyuan， in which the deficiency of spleen-stomach qi， deficiency of original qi and disorder of qi and fire are the pathological basis of the disease.The case of deficient fire flaring up，upward disturbing the throat along pathway of the channels and getting into the mouth and tongue causing lesions is the pathogenesis of the disease.The treatment of it should give priority to supplementing the spleen and stomach and purging yin fire，supplemented with clearing heat and draining dampness，and resolving stasis and toxin.The above understandings provided new ideas for treatment of syndrome differentiation for diabetes mellitus complicated with oral ulcer.