The photonic crystal (PC) flat lens has broad application prospects in the field of super-resolution imaging and it has been researched deeply, but the lenses studied in the past are only suitable for specific polarization mode or specific incident light source. To this end, we propose a kind of PC flat lens with scatterer-size gradient. This flat lens can simultaneously realize point source imaging and plane wave focusing in TM and TE polarization modes, and break through the diffraction limit in TM polarization mode, and realize sub-wavelength imaging and focusing in TE polarization mode. This flat lens can realize polarization-insensitive imaging and focusing without any additional polarization components. It is expected to be used in the design of multi-functional optical polarization-insensitive imaging and focusing devices, and can be applied to real-time biological display, high-density optical storage and microelectronic lithography, and improve the application potential of gradient PC flat lens.