To analyze the phenomenon related to the detection of explosives by laser illumination and imaging, theoretical analysis of the process of the laser illumination on the target is carried out firstly. The simulation is implemented with the software ANSYS. Three- dimensional models are built for the study of temperature distribution. A CO2 laser is used with expanded beam to irradiate the target (3 m away), and obtain thermal images (8~14 μm) of the targets with TNT and RDX residues. Studies show due to the effective radiation of explosive and background is determined by the emissivity and reflectivity in 8~14 μm waveband when laser irradiation at a certain power, the explosives and the substrate have significant difference in thermal images. Moreover, irradiation power and time difference may also be important factors affecting the detection.