Method of preparing oligosaccharide using biomass sugar as raw material

A method of preparing an oligosaccharide using a biomass sugar as raw material, the method comprises the following steps:

• (1) placing the biomass sugar in a quartz tubular reactor, then passing a high-purity nitrogen gas to replace air in the quartz tubular reactor, setting a temperature of a tubular thermal conversion reactor to 300° C. to 700° C., when the temperature in the tubular thermal conversion reactor reaching the set temperature and being stable, placing the quartz tubular reactor at a temperature-controlled zone of the tubular thermal conversion reactor, controlling a thermal conversion time to 10 seconds to 130 seconds, and then placing the quartz tubular reactor in a cooling liquid or air to quench, so as to obtain an oligosaccharide precursor;
• (2) dissolving the oligosaccharide precursor obtained in step (1) by adding a pure water, so as to obtain an oligosaccharides-rich solution;
• (3) purifying the oligosaccharide-rich solution obtained in step (2) through an inorganic ultrafiltration membrane, so as to obtain an oligosaccharide solution having a molecular weight within 1500;
• (4) drying the oligosaccharide solution obtained in step (3), so as to obtain the oligosaccharide.