Four polysaccharides were obtained by hot water extraction and alcoholic precipitation with different volume fractions of ethanol (30%, 50%, 70% and 80%), and were recorded as BLP-30, BLP-50, BLP-70 and BLP-80 respectively. The structural characterization of the polysaccharides, such as mass fraction of soluble total sugars, protein amount, mass fraction of glyoxylate, UV absorption spectra, monosaccharide composition and relative molecular masses, were determined and correlation analysis was applied to compare the inhibition ability of the polysaccharides against α-amylase and α-glucosidase with the structural characterization. The structural characterisation of total soluble sugars mass fraction, protein quantity fraction, glyoxylate mass fraction, UV absorption spectra, monosaccharide composition and relative molecular mass in BLP-30, BLP-50, BLP-70 and BLP-80 were determined and correlation analysis was applied to compare the inhibition ability of polysaccharides against α-amylase and α-glucosidase with the structural characterisation. The results showed that all four Lanzhou lily polysaccharides contained no or very small amounts of glyoxalate and protein, and the highest total soluble sugar mass fraction was 30.96% for BLP-80. The four polysaccharides are mixed polysaccharides, all consisting of mannose, glucose and a small amount of galactose, with a gradual decrease in relative molecular mass from 645 kDa for BLP-30-1 to 2.45 kDa for BLP-70-4. BLP-30 and BLP-50 are glucans, BLP-70 is galactomannan glucans and BLP-80 is mannoglucan glucans, 0.49%; BLP-70 mannose, glucose and galactose molar fractions were 26.94%, 68.21% and 4.85, respectively; BLP-80 mannose and glucose molar fractions were 41.56% and 58.44%, respectively, without galactose. All the four polysaccharides had certain in vitro hypoglycemic activity and showed a quantitative-effect relationship. The four polysaccharides inhibited α-amylase and α-glucosidase in the following order: BLP-70 > BLP-80 > BLP-30 > BLP-50 and BLP-50 > BLP-70 > BLP-80 > BLP-30, respectively. Correlation analysis showed that the inhibitory ability of the four polysaccharides on α-amylase was positively correlated with the molar fraction of mannose and galactose (P< 0.05), and the inhibitory ability of the polysaccharides on α-amylase was negatively correlated with the molar fraction of glucose (P< 0.05); the inhibitory ability of the polysaccharides on α-glucosidase was negatively correlated with the mass fraction of total soluble sugars, the mass fraction of glyoxylate, and the molar fraction of mannose (P< 0.05), and the inhibitory ability of polysaccharides on α-glucosidase was positively correlated with the molar fraction of glucose (P< 0.05). The in vitro hypoglycaemic activity of BLP-70 was stronger among the four species of Lily of Lanzhou polysaccharides. It is assumed that the relative molecular mass of Lily of Lanzhou polysaccharides is between 2 and 3 kDa, and that the ratio of mannose to glucose is between n (mannose):n (glucose) = 1:2.5, which is more conducive to the in vitro hypoglycaemic activity of Lily of Lanzhou polysaccharides.