Lens is usually immersed into oil or other mediums to increase the numerical aperture of the focusing lens and significantly improve the focusing efficiency. V-shaped plasmonic lens has attracted many research attentions because of its advantages like ability of reduced spherical aberration, ultrathin thickness, simple design, and high adjustability. However, it is hard to apply in practice since the focusing ability of such lens is low and errors can be great. Based on the incident light which wavelength is 1.5 mu m, a V-shaped plasmonic planar lens with a thickness of 30 nm is designed, and it is immersed into the medium at a refractive index of 1 to 2.5. By matching conditions of transverse wave vector and Rayleigh limit formula, when the refractive index of mediums increases, the focusing efficiency becomes stronger, the spatial resolution is higher, and the focusing spot is changed. Through this easy-to-handle method, the significant focusing effect can be obtained, and this method promotes the applications of planar focusing lens in optical focusing, optical storage, laser-writing, and miniature optical integrated devices.