The invention provides an Ascophyllum Nodosum oligosaccharide preparation method and application of Ascophyllum Nodosum oligosaccharide to medicines for reducing blood sugar. The preparation method is characterized by including a technological process of taking degreased, deproteinized and depigmented Ascophyllum Nodosum fucoidin as a raw material, combining microwave-assisted treatment, adding pectinase, cellulose, fucoidanase and sulfatase to subject the fucoidin to enzymolysis, removing the fucoidin not degraded sufficiently and zymoprotein through ethanol precipitation, centrifuging liquid supernatant prior to distillation and concentration, blending the distilled and concentrated liquid supernatant with oligosaccharide generated during polysaccharide extraction prior to sieving by a molecular sieve, and subjecting interception matters to freeze drying so as to obtain the Ascophyllum Nodosum oligosaccharide, wherein ethanol used during preparation is recycled. Through in-vitro and cell tests, the Ascophyllum Nodosum oligosaccharide has a remarkable improvement effect on glycometabolism. The preparation method is an efficient and environment-friendly technological method for preparing the Ascophyllum Nodosum oligosaccharide by microwave-accelerated enzymatic digestion.