The　noises　of　a　special　gas　IR　detector　fabricated　by　MEMS　mainly　include　the　temperature-fluctuation　noise,　mechanical-thermal　noise　and　the　background　noise.　Based　on　thermal　conduction　theory,　a　thermal　model　is　built　up　to　get　the　effective　thermal　content　and　the　effective　thermal　conductivity　to　be　1.010-3　W/K　and　8.1　μJ/K,　respectively,　and　the　temperature-fluctuation　noise　is　nearly　1.7310-10　W/Hz1/2.　According　to　the　device　operating　principle　and　energy　equipartition　theory　of　thermodynamics,　the　mechanical-thermal　noise　is　9.9610-9　W/Hz1/2,　and　the　background　noise　is　nearly　3.2210-11　W/Hz1/2,　so　that　the　normalized　detectivity　is　deduced　to　be　9.03106　cmHz1/2/W.　Experiment　results　show　that　the　mechanical-thermal　noise　coming　from　the　structure　and　the　mechanical　performance　of　a　heavily　doped　Si　membrane　is　the　main　noise　of　the　device,　which　can　be　reduced　by　minishing　the　membrane　area　and　extending　the　membrane　thickness　to　increase　the　mechanical　resonant　frequency　of　the　membrane,　but　should　be　at　the　price　of　reducing　the　sensitivity　of　the　device.　The　vibration　of　environment　also　influences　on　the　device　a　lot.　To　decrease　the　noise　coming　from　the　fluctuation　of　air　pressure　and　the　temperature　fluctuation　of　the　surrounding,　a　new　double-cell　counteracting　structure　is　presented.