The disclosure discloses a preparation method of outer core octasaccharide of Helicobacter pylori lipopolysaccharide, and belongs to the field of carbohydrate chemistry. The structure of the outer core octasaccharide of Helicobacter pylori is (α-D-Glc-(1-3)-α-D-Glc-(1-4)-β-D-Gal-(1-7)-D-α-D-Hep[α-D-Glc-(1-6)-α-D-Glc-(1-6)-α-D-Glc-(1-2)-D-α-D-Hep]-Linker. The structure consists of three monosaccharides: glucose, galactose and heptose. It contains five α-glycosidic bonds of glucose, one β-glycosidic bond of galactose and two α-glycosidic bonds of heptose. The disclosure prepares the octasaccharide by the synergistic action of remote neighboring group participation effect, solvent effect, temperature effect, additives and leaving groups. The reducing end of the octasaccharide may also be linked with a linker for future attachment to the protein to form a glycoconjugate for immunological studies.