Spectral image degradation is a common problem of acousto-optic tunable filter (AOTF) imaging spectrometers, and it is difficult to quantitatively predict. To quantitatively evaluate the image quality of spectral images, a modulation transfer function (MTF) calculation method including the influence of the AOTF rear cut angle is proposed. In this method, the line spread function (LSF) of the AOTF device is obtained by establishing the spectrum-space dimension response model of the AOTF, and then the theoretical MTF is obtained by Fourier transform of the line spread function. In the validation experiment, the deviation between the measured MTF value and the theory is less than 15% within the cut-off frequency. It is a theoretical basis for image quality evaluation of the AOTF. Furthermore, the influence of different AOTF rear cut angles on spectral image quality is discussed by this quantitative method. After simulation, it is concluded that the rear cut angle of the AOTF has difficulty meeting the requirements of lateral chromatic aberration and image sharpness at the same time. The cut angle needs to be selected according to the requirements of practical application. Therefore, it is significant to evaluate the image quality of AOTFs with different rear cut angles. The method is an important theoretical basis for AOTF device design.