CO2　laser　direct-writing　ablation　micromachining　technique　is　used　to　fabricate　the　microchannel　on　the　polymethyl　methacrylate　(PMMA)　substrate.　The　correlation　between　the　process　parameters　(the　beam　translational　velocity　and　the　processing　number　of　times)　and　the　micromachining　quality　(the　width　and　the　depth　of　the　microchannel)　is　investigated.　For　a　given　processing　times,　the　depth　and　width　of　microchannel　both　decrease　along　with　the　enhancement　of　beam　translational　velocity.　At　fixed　beam　translational　velocity,　the　depth　and　width　are　direct　proportion　to　the　processing　times,　respectively.　The　microchannel　is　fabricated　on　different　PMMA　substrates　with　smooth　surface,　rough　surface　and　surface　adhering　water.　The　micromachining　quality　and　the　surface　roughness　of　the　microchannel　are　compared.　The　microchannel　with　a　hydraulic　diameter　of　80　μm　is　fabricated　under　two　experimental　conditions,　and　the　comparative　experiment　has　been　performed　on　the　microchannel　with　the　relative　roughness.　It　is　found　that　the　relative　roughness　is　minished　effectively　when　a　laser　is　used　to　make　ablation　of　the　microchannel　on　the　PMMA　substrate　with　surface　adhering　water.　The　improved　fabrication　procedure　will　be　used　to　increase　the　availability　of　CO2　laser　direct-writing　ablation　technique　on　the　micromachining　of　the　PMMA　microfluidic　chip.