Starting from the propagation law of the crossspectral density function, an analytical expression for the onaxis spectrum of spherically aberrated pol y chromatic Gaussian beams diffracted at an aperture is derived, the aberration f ree result is obtained as a special case treated in the paper. Detailed numerica l calculations and physical analysis are presented. It is shown that the onaxis spectrum in the near field may be redshifted and blueshifted in co mparison wi th the aberrationfree case, and the spectral switch may also take place. In pa r ticular, the influence of spherical aberration on the behavior of spectral swit ches is illustrated.