The invention relates to a method for preparing reducing sugar by using supercritical/subcritical water-CO2 hydrolyzed bagasse pith. The method comprises the following steps: (1) adding bagasse pith and water into an autoclave, filling CO2, carrying out reaction for 1-60 minutes under the conditions that the temperature is 160-260 DEG C, the pressure is 1-8MPa and the stirring rotating speed is 100-500r/min, and then, filter-pressing material in the autoclave and taking out the filtrate, namely the reducing sugar, wherein the filter residue is left in the autoclave; (2) introducing water and CO2 into the autoclave with the filter residue in the step (1) to carry out reaction continuously for 0-50 minutes under the conditions that the temperature is 370-400 DEG C, the pressure is 20-27MPa and the stirring rotating speed is 100-500r/min and then cooling the material in the autoclave to room temperature, carrying out filter pressing, wherein the obtained filtrate is reducing sugar. The method provided by the invention is used for improving the yield of hydrolyzed bagasse pith, is free of either solvent pollution or complex post-treatment steps and is fast in reaction rate and high in yield. The yield of reducing sugar is over 90%.