The effect of primary aberration and high-order aberration, especially spherical aberration, on imaging of flat-field holographic concave gratings is analyzed by using geometrical aberration theory. The difference between design results when spherical aberration is corrected or not is discussed through a specific design case, for situation of wide wave-range and narrow wave-range, respectively. Analysis is made through comparison of the spectral image size and the geometrical aberration coefficients. It is discovered that, for wide wave-range grating, the image size is determined mainly by primary aberration. Spherical aberration can be ignored during grating design procedure because the effect of spherical aberration is much less than that of primary aberration. But in the case of narrow wave-range grating, the effect of spherical aberration becomes significant because the defocus aberration is very small. The spherical aberration must be corrected in the design procedure when a much better imaging quality is expected.