A new method for designing narrowband spectral high reflectivity filters, which combines ultrathin metallic film with unsymmetrical Fabry Perot interfere filter, is proposed in this paper. The reflection filter utilizes the metallic film material that has a property of n≈k. The layer structure in visible spectrum is given. A few of formulas of the spectral reflection properties, such as reflectivity, maximum reflectivity and reflection bandwidth, is derived and is proved by experiment. A method of designing narrowband spectral high-reflectivity filter in non-visible spectrum is provided.