A modified Czerny-Turner spectral imaging system was developed based on aberration theory to minimize the large astigmatism in classical Czerny-Turner spectrometers. The astigmatism from a plane grating placed in the divergent light beam was used to compensate the astigmatism from an objective lens. The broadband astigmatism corrected simultaneously conditions were deduced, and the astigmatism was corrected in a wide spectral region. The principle and method of astigmatism correction were analyzed in detail, and the initial parameter computing was programed. As an example,a Czerny-Turner imaging spectral system operating in 540-780 nm was designed. The ray tracing and optimization for the spectral imaging system were performed with ZEMAX-EE software. The analyzed results demonstrate that the total field-of-view modulation transfer function is higher than 0.52 in the whole working spectra.The system shows good imaging quality due to the astigmatism to be corrected in the wide spectral region synchronously.Obtained results prove the feasibility of the modified method.