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 1.0×10^-3 W/K and 8.1 μJ/K,respectively ,and the temperature-fluctuation noise is nearly 1.73×10^-10 W/Hz^1/2. According to the device operating principle and energy equipartition theory of thermodynamics,the mechanical-thermal noise is 9.96×10^-9 W/Hz^1/2,and the background noise is nearly 3.22×10^-11 W/Hz^1/2,so that the normalized detectivity is deduced to be 9.03×10⁶ cm·Hz^1/2/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.