An analytical expression for the on-axis spectrum of spherically aberrated polychromatic partially coherent light diffracted at an aperture was derived based on the propagation law of the cross-spectral density function. Spatially fully coherent result was obtained as a special case. The influence of spherical aberration on the behaviors of spectral switches was stressed. It was shown that spherical aberration may induce differences of spectral shift and changes of the critical position.