The invention relates to an extraction method of aucubin and an application of the aucubin. The method comprises the simple steps of crushing plantain, soaking in alcohol overnight, conducting ultrasonic extraction, merging, recovering, concentrating and centrifuging an extracting solution, obtaining supernate, adopting macroporous resin and eluting an alcohol solution, conducting thin-layer chromatography detection, collecting and concentrating fraction at a target point, conducting silicagel column chromatography on a concentrate, and then collecting and concentrating the fraction at the target point, repeating the silicagel column chromatography to obtain macrocrystalline, purifying a crude product through high performance liquid chromatography, collecting target components, freezing and drying, and finally obtaining white amorphous powder. The substance extracted from the plantain by the method is the aucubin. The aucubin can be used for treating and preventing diabetes mellitus and concurrent encephalopathy of the diabetes mellitus, the occurrence and development mechanisms of the diabetes mellitus and complications of the diabetes mellitus are further deeply studied, and new-generation drugs for resisting the diabetes mellitus and the complications of the diabetes mellitus can be developed.