Sugarcane　bagasse　microcrystalline　cellulose　(SBMC)　was　first　prepared　under　subcritical　Water/CO2　by　degradation　of　sugarcane　bagasse　cellulose　(SBC).　The　obtained　products　were　characterized　by　FT-IR,　XRD　and　TGA　and　the　results　showed　that　the　amorphous　parts　of　SBC　was　easily　decomposed,　and　yet　the　crystal　parts　kept　a　good　form　all　along　during　the　degradation,　which　demonstrated　SBMC　had　good　thermal　stability.　Furthermore,　the　reaction　temperature,　reaction　time,　reaction　pressure　and　liquid-solid　ratio　were　systematically　investigated　during　the　work.　The　optimum　reaction　conditions　are　as　follows:　the　reaction　temperature　was　200　degrees　C;　the　reaction　time　was　60　min;　the　reaction　pressure　was　2　MPa　and　the　liquid-solid　ratio　was　40:1.