A novel structure of a metal/dielectric/metal (Ag/SiO2SiO2/Ag) sandwich nanocrescent has been proposed and studied. We make a detailed numerical analysis on the extinction efficiency and LSPR property of the sandwich nanocrescent by using the finite difference time domain (FDTD) method. It clearly demonstrates that a comparable field enhancement can be achieved by varying the thickness of the SiO2SiO2 layer at different incident polarizations. Excited in the Y-polarization, the maximum electric field enhancement factor reaches 600 at the peak wavelength 1108.9 nm, which is six times higher than previous reported single layer nanocrescent. The refractive index sensitivity of this new sandwich nanocrescent is 375.5 nm/RIU (refractive index unit). The structure is shown to produce a high local field enhancement as well as wide plasmon resonance tunabilities. Besides, compared with adjusting the shape and size of the single layer nanocrescent structure, it is much more convenient and easier to change the thickness of the sandwich nanocrescent. Due to its excellent properties, this structure is very suitable for LSPR and SERS nanosensing substrate.