We propose a method for a high-power tunable sub-nm narrowband near-diffraction-limited superfluorescent fiber source (SFS) based on a single-lens spectral filter (SLSF), which is composed of a grating monochromator and a movable space-fiber coupler. The center wavelength of the SLSF is set to 1063 nm with a full width at half-maximum (FWHM) linewidth of less than 0.08 nm. By utilizing the SLSF and a broadband amplified SFS, a tunable sub-nm narrowband SFS seed source is obtained, and the central wavelength of the sub-nm narrowband SFS seed source can be easily tuned from 1052.4 nm to 1072.8 nm by adjusting the SLSF. After a three-stage amplifier system, the output power of this sub-nm narrowband SFS is boosted to 230W with the FWHM range from 0.10 nm to 0.12 nm. The beam quality factors (M-2) of the full power sub-nm narrowband SFS at 1060.1 nm is 1.20. The proposed tunable sub-nm SFS has advantages of high spectral resolution, simple configuration, which may have potential applications in the industrial production and scientific research.