The on-axis irradiance distribution behind the cylindrical lens with spherical aberration illuminated by partially coherent light is derived from Collins formula. The effect of the Fresnel number and spatial coherence of the incident light as well as the spherical aberration of the lens on the on-axis irradiance distribution is also investigated by numerical simulation. It is shown that,when the light with low Fresnel number or low spatial coherence by the aberration-free lens,the focal shift appears. Besides,the focal shift also appears when light is focused by lens with spherical aberration.