We studied how a protection layer influences the performance of an Ag-coated dielectric tip in tip-enhanced Raman spectroscopy using 3D simulations. Two typical chemically inert materials, Au and SiO2 were considered in this work. Neither of them introduces a significant shift of the resonance frequency; the field enhancement as well as the spatial resolution can also be preserved with a thin protective layer. Surprisingly, a 5 nm layer of SiO2 can even improve the field enhancement of the tip. This provides a new method to prevent Ag tips from undesired physical and chemical damages without losing their original performance for TERS.