Czerny-Turner(C-T)spectrometer usually been used monochromators and spectral radiometers due to its compact structure and spherical mirrors. However, astigmatism greatly confines C-T spectrometer used in imaging spectrometers. Astigmatism-free methods were analyzed theoretically and verified experimentally under non-parallel grating illumination. As a result, astigmatism of C-T spectrometer can be corrected under convergent or divergent grating illumination. When the cosine square ratio of grating incidence to difference angle was more than 1, divergent illumination can correct astigmatism. Conversely, when it was less than 1, convergent illumination can correct astigmatism.