The invention relates to a method for catalytically degrading bagasse cellulose and preparing microcrystalline cellulose by carbon dioxide, which comprises the following steps of: using bagasse cellulose pulp obtained in a way that utilizing bagasse to making pulp so as to make paper and deionized water as raw materials and taking subcritical water as reaction mediums and CO2 as a catalyst. The method has the beneficial effects that the process is simple, the source of the used raw materials is wide, the cost is low, the polymerization degree of the obtained bagasse microcrystalline cellulose is 50-300; products are clean and are not polluted, three wastes are not discharged in the whole process, and the significance is important in view of energy conservation, emission reduction and atom economy.