We propose and present a planar plasmonic lens formed by an array of spatially varying sub-wavelength rectangular annular patterned in the upper Au film of a metal-insulator-metal (MIM) structure. It is found that the reflected phase and amplitude can be well controlled by manipulating the width of the annular gaps and the length of the MIM cavity, in which localized surface plasmonic resonances occur. A reflective planar plasmonic lens that can generate a spherical wave-front in the reflected field has been realized through an optimized design at wavelength 1.55μm. Numerical results using the Finite Difference Time Domain (FDTD) method show that the focal length can be precisely controlled with a beam spot size at focal plane being close to the diffraction limits, and the focusing efficiency is up to 50%. It provides a great potential for applications in advanced nanophotonic devices and integrated photonic systems.