We have proposed an all-dielectric polarization-independent metalens, which can be used for sub-wavelength focusing. The phase discontinuity of transmission light is produced by varying the diameter of the nanopillar. In this way, the incident light can be focused into sub-wavelength spot. Besides, finite-difference time-domain results show that near diffraction limit focusing is realized for the wavelength from 580 nm to 750 nm, which means the proposed metalens possesses a broad operation bandwidth. This all-dielectric metalens features polarization independent and broad bandwidth, and thus promises great potential for the applications of nanolithography, dense storage and biophotonics.