The magnification changing with the incident wavelength, which would bring the different spectral image matching error, is inevitable in the multispectral infrared imaging spectrometer using binary optical element (BOE) as spectro"element because its focal length changes with incident wavelength. To improve the optical performance of the multispectral infrared imaging spectrometer, the hybrid refraction/ diffraction zoom optical system combining the off"axial three"mirror system is firstly put forward. The front element of multispectral infrared imaging spectrometer is an unobscured and afocal three "mirror telescope,which not only makes for enlarging the clear aperture of multispectral infrared imaging spectrome" ter,but also reduces the size and weight.The rear elements of multispectral infrared imaging spectrometeris a hybrid refractive/diffractive three#elements zoom optical system, which eliminates the different spec-tral image matching error in the multispectral infrared imaging spectrometer using BOE aThe magnification changing with the incident wavelength, which would bring the different spectral image matching error, is inevitable in the multispectral infrared imaging spectrometer using binary optical element (BOE) as spectro"element because its focal length changes with incident wavelength. To improve the optical performance of the multispectral infrared imaging spectrometer, the hybrid refraction/ diffraction zoom optical system combining the off"axial three"mirror system is firstly put forward. The front element of multispectral infrared imaging spectrometer is an unobscured and afocal three "mirror telescope,which not only makes for enlarging the clear aperture of multispectral infrared imaging spectrome" ter,but also reduces the size and weight.The rear elements of multispectral infrared imaging spectrometeris a hybrid refractive/diffractive three#elements zoom optical system, which eliminates the different spec-tral image matching error in the multispectral infrared imaging spectrometer using BOE as spectro element The designed system is very simple in structure. Moreover, the modulation transfer function (MTF) of the designed system is greater than 0.3 for 20 c/mm spatial frequency, which meets the resolution requirement of the stare array detector of the multispectral infrared imaging spectrometer. In addition, the image size of the designed system is 7.2 mm, which meets 25 μm detector cell and 128 ×128 infrared detector arrays.