The invention discloses a milkvetch root extract. The milkvetch root extract contains astragaloside IV the content of which is no less than 0.08mg/ml; the invention also provides a preparation method of the milkvetch root extract, which extracts the active constituent of astragalus membranaceus by a water ethanol method; and in addition, the invention also provides an application of the milkvetch root extract in preparing medicaments which prevent and control diabetics from damage caused by retroegulation of hypoglycemia hormone. Animal experiments indicate that: the active constituent of the astragalus membranaceus in the invention has the function of reconstructing part of hormone reactions of HAAF rat model, can activate sugar-sensitive parts such as PVN, NTS and the like and CRF nerve-corpuscle in a brain, thereby improving the early warning mechanism of an organism to hypoglycemia. The invention has a wide prospect of curing one of the complications of diabetes mellitus, i.e. HAAF.