Multispectral imaging of the infrared target provides effective method for the study of infrared radiation characteristic. It has significant applications in many fields of detection technologies such as material identification, false-target discrimination, complex backgrounds suppression. Based on the blackbody radiation theory, a new spectral imaging method, which extends the spectral band of measured infrared image, is proposed. Firstly, the infrared thermal image is collected by the mid-wave infrared thermal imager (3～5 μm). The temperature value of different pixel of the thermal image is obtained by calculating the radiation. Then the corresponding radiation of the different pixels in the desired spectral band is acquired from the temperature through Planck formula. And finally the radiation data are quantified and represented as multispectral infrared images.