Aiming at the problem of secondary diffraction in aberration correction of an M-type Czerny-Turner spectrometer, we propose the anti-secondary diffraction condition for this optical structure under the coma-free condition. The main factors of secondary diffraction are analyzed and deduced theoretically. Combining the anti-secondary diffraction condition with the constraint coma-free condition, we built an M-type coma-free Czerny-Turner spectrometer. In order to verify the correctness of the theoretical analysis, the Zemax OpticStudio simulation software is used for a comparative experiment. The simulation results show that there is no secondary diffraction in the M-type optical path structure meeting the constraints, but there is secondary diffraction in the M-type optical path structure that does not meet the constraints, which is consistent with the theoretical analysis result. Under the condition that the incident slit width is 25 μm and the grating engraving line density is 600 line/mm, the spectral resolution of the optical system optimized by the software is better than 1 nm in the range of 200?500 nm.