Based on the principle of acousto-optic tunable filter (AOTF), the optical system of an imaging spectrometer is designed. The spectral range of the system covers 0. 44～0.78 μm. An even aspheric surface is used to improve the imaging quality, optical transmissivity and simplify the lens structure. A telecentric optical arrangement is designed using a doublet lens for the fore-optical system. And for the rear imaging lens, a triplet lens is used which contains an aspheric surface. The aspheric surface and its location are optimized based on the primary aberrations theory of aspheric. The system takes the +1 order diffracted beams for imaging. The modulation transfer function (MTF) of the optical system is better than 0.6 at the spatial frequency of 32 lp/mm and the tolerance is moderate.