In this Letter, a photonic crystal (PC) flat lens with a scatterer-size gradient is proposed, which simultaneously achieves imaging of the point source and sub-wavelength focusing of the plane wave in the first, second, and fifth bands. The imaging of the point source breaks through the diffraction limit in the second and fifth bands. The PC flat lens with the scatterer-size gradient is expected to be used in a new multifunctional optical imaging and focusing device, which improves the application potential of a PC flat lens.