  Recent works [PhysRevLett.125.170402 (2020) and PhysRevLett.125.043603
(2020)] verify that the perfect single-photon character is a global property of
resonance fluorescence including all the spectral components, and then the
absence of some components can spoil the single-photon character, which
considerably limits the application of filtering scheme to obtain single-photon
source (SPS) with subnatural linewidth. Here, we note that the linewidth of
fluorescent photon is not always limited by the intrinsic linewidth of the
emitter, and provide a scheme where the linewidth of the fluorescent single
photon can be much smaller than the natural linewidth of emitter by
manipulating external coherent fields. By simulating the response of the filter
or detection setup with a bandwidth approaching and even much smaller than the
natural linewidth of emitter, the realization of ultranarrow SPS can be
confirmed. Besides, we demonstrate that this scheme of ultranarrow SPS can be
implemented based on various real physical platforms.
