The invention provides a differential spectral imaging method. The method is characterized by comprising the steps of dividing a tunable filter penetration peak into multiple sections on the basis of obtaining a tunable filter penetration rate function, namely differentiating a spectrum within a bandwidth, obtaining an integral equation containing multiple unknown numbers at every scanning time at the spectral imaging by adopting each section of spectrum as an unknown number, scanning and imaging for multiple times by adopting a step length which is smaller than the bandwidth so as to obtain an integral equation group; subducting surplus unknown spectrums in the equation group by utilizing a cut-off filter plate, so that the unknown numbers of the equation group is as many as equations, solving the linear equation group through integral transformation and error compensation, and obtaining a more precise spectrum structure of the tunable filter plate within the bandwidth interval. The spectral resolution capacity at the differential spectral imaging can be controlled by classifying the bandwidth of the filter, so that the narrow-band filter can be realized by utilizing the width filter, and the spectral resolution capacity and application range of a spectral imaging instrument can be greatly increased.