After using distributed infrared sensors to detect the same target, the image of the multi-sensor target can be fused at pixel level to improve the signal-to-noise ratio (SNR), so as to improve the detection ability of the system. Starting with the detection range of a single infrared sensor, the range evaluation model of distributed multi-infrared system collaborative detection is described, and the noise equivalent temperature difference (NETD) formula is deduced including integral time. The Little Hudson detection range equation is expressed by NETD. The multi-sensor image fusion method is used to improve the SNR of the image, which is equivalent to extending the integration time and suppressing noise to improve NETD. Combined with the relation formula of the false alarm probability and detection probability, a reference for setting SNR in the detection range equation of multi-infrared sensors is provided. NETD and SNR are considered in the detection range evaluation model of distributed multi-infrared systems. Theoretical calculation shows that multi-sensor cooperative detection can effectively improve the detection range of the system, which provides important theoretical support for the comprehensive demonstration of distributed infrared systems.