Asteroid exploration has become one of the important ways to understand the origin of the universe, exploit the space mineral resources and protect the earth from the asteroid impact. The imaging spectrometer, which can acquire the spatial and spectral information simultaneously, has become one of the most important payloads for the asteroid exploration. In this paper, the optical design of a broadband Offner imaging spectrometer for the asteroid exploration is presented. It covers from 0.4μm to 3μm, and the F/# is 6 and 3 for the VNIR band (0.4μm-1.0μm) and the SWIR band (1.0μm -3.0μm), respectively. The convex grating in the imaging spectrometer is divided into two regions of different groove spacing, and each region is optimized with different blazed angles to improve the spectral response of this imaging spectrometer. The diffraction efficiency is analyzed with the Comsol software and the result is greater than 0.36 in the wavelength range of 0.4μm to 3μm. After optimized with the optical design software, the diameter of the imaging spectrometer's spot diagram is less than one pixel, and the lowest MTF is 0.460 at the Nyquist frequency. The smile and keystone is less than 10% of the pixel.