Abstract: Compared with spectral imager based on the flat or concave grating, the Offner spectral imager which utilizes convex grating as the diffractive element has several advantages, such as large numerical aperture, no Smile, small spectral Keystone, compact volume, easy manufacturing and alignment, etc. Consequently, it's becoming widely applied in many fields. In order to meet the requirements of highly integrated, broad working spectrum and miniaturized payload for deep space exploration mission, in this article, an Offner spectral imager with dualband was designed. At the same time, a neatly spectral splitting method using different part of grating was implemented. The system which met the design requirements had fine image quality, small volume and compact configuration. All reflective curves of the optics were designed sphere, which had loose tolerance and were convenient for manufacturing and alignment. The convex grating was framed and its diffraction efficiency was analyzed. The result shows that the dynamic range of the whole system can be enlarged effectively using the elaborate design.