The development of uncooled IRFPA pixel structure from a single-layer structure to a double-layer structure reduces noise and improves performance. The pixel structure and its main physical parameters are introduced. It is noted that the main difference between the double-layer structure and the single-layer structure lies in the difference in the effective area of the pixel and thermal conductivity of the bridge leg. The 3-D noise model is an effective method to analyze IRFPA noise, in which spatio-temporal random noise is the main noise part of uncooled IRFPA. The mechanism of spatio-temporal random noise in uncooled IRFPA is analyzed, and a spatio-temporal random noise model is established. The relationship between spatio-temporal random noise, the effective area of the pixel, the thermal conductivity of the leg is obtained. An uncooled IRFPA with a single-layer pixel structure is improved by changing it into a double-layer pixel structure, and the noise is measured. The measured data proved the effectiveness of the uncooled IRFPA spatio-temporal random noise model.