Optical metasurfaces are ultrathin planar nanostructures that manipulate wavefronts and polarization of light. Here, we propose a step-zoom doublet lens of dual focal lengths composed of two dielectric metasurfaces at 1550 nm wavelength. The highly integrated doublet lens can operate with long and short focal length modes by easily converting the handedness of the incident circular polarized light, meanwhile the focal plane remains unchanged. Two tight focal spots 0.57 lambda and 0.79 lambda are achieved with corresponding numerical aperture of 0.95 and 0.66, respectively. Compared with a conventional zoom system, this step-zoom doublet lens is ultrathin, compact and flexibility. The ultra-compact doublet lens can be applied in zoom imaging, infrared detection and on-chip photonics components.