An ultrathin, planar, broadband metalens composed of metal rectangular split-ring resonators (MRSRRs) has been designed, which shows dual-polarity characteristics for different types of circularly polarized (CP) light incidence. The designed metalens can be considered as the focusing lens and the diverging lens under left-handed CP and right-handed CP light incidence, respectively. The phase discontinuity of the cross-polarized transmission light is produced by optical-axis rotation through modulating two arms’ lengths of the MRSRR. The MRSRR metalens possesses a wavelength-controllable focal length and a relatively larger chromatic aberration compared with the conventional lenses. And the focal length changes from 9 to 7 μm with incident wavelength from 740 to 950 nm. The dual-polarity flat metalens opens a door for new applications of phase discontinuity devices, and it will promote the fabricating capability of on-chip or fiber-embedded optical devices.