The uncooled IR imaging technology has wide applications. Using the optical readout uncooled IR imaging system built in our laboratory, thermal images of human body are obtained successfully by placing a bi-material micro-cantilever array (FPA) without silicon substrate in air. By comparing the IR imaging results obtained by placing FPA under different vacuum pressures, the pressure dependence of the thermal conductance and system noise is modeled, and the influence to the system performance is analyzed. The model of thermal conductance of gas is analyzed and verified experimentally on condition that the mean-free path of the gas molecules is larger than the gas gap between the bi-material cantilever and the wall of vacuum chamber. Experimental analysis indicates that for FPA working in air, the random vibration noise of the cantilever impacted by the gas molecules' thermal motion is the main source of the system noise. When the pressure decreases to lower than 10-2 Pa, the influence of gas conductance can be neglected, the total thermal conductance decreases to the limit of radiative conductance, and the random vibration noise of the cantilever is reduced to a minimum. The experimental results and theoretical analysis agree well.