Thermal characteristics of new substrate-free FPA (focal plane array) with full hollow supporting frame structure are very different from that of traditional substrate FPA. The thermal analysis model which based on temperature-constant substrate assumption is not efficient any more. Therefore, an equivalent circuit model was presented to analyze the thermal response characteristics of substrate-free FPA with electrics and holistic approach. According to this model, the thermal characteristics of substrate-free FPA under non-vacuum environment were analyzed. The result shows that, the substrate-free FPA has excellent infrared imaging performances under atmospheric pressure, while its NETD (noise equivalent temperature difference) is just increased several times than in the vacuum environment.