Abstract: Obtaining spatially high-resolution optical information effectively through overcoming the restriction of the diffraction limit in the optical field system has become an important research topic in the area of nanophotonics. We applied finite difference time domain (FDTD) method to study the characteristics of the focused spot nanometer jet formed by a core shell structure with various core refractive index and core radius. Results show that under the circumstance of the core radius unchanged, while the core had low refractive index, the incident energy was concentrated in the boundary region of the core and the shell layer, at the same time, large amount of energy injects into the side lobe; when the refractive index of the core was larger than that of the outer shell, double enhanced points at the end of the core shell structure could be formed; as the cores relative refractive index was unchanged, active control of internal and external core focus point location by changing the core radius was achieved.