When detecting a target over the diurnal cycle, due to the large temperature variation of targets and background objects, the infrared image contrast between target and background in a scene would probably approach to zero, by which the target would be blended into the background in thermal infrared image, causing the blindness of thermal infrared system, lowering the detection accuracy and even making the thermal sensor lose the target. Concerning this issue, based on the brief theoretical analysis on how the thermal crossover over the diurnal cycle influenced a conventional thermal infrared system, a mid-infrared(3-5 μm) multi-spectral method using different infrared spectral characteristics of different targets to solve the issue was presented. Furthermore, the prototype design and how multispectral technology was employed to help solve the thermal crossover detection problem were also described. The experimental results show that the multi-spectral infrared imaging system can increase the contrast of the detected images by 18% compared to the MWIR broadband image and effectively solve the failure of the conventional infrared sensor during the diurnal cycle, which is of great significance for infrared surveillance applications.