An on-chip ultra-narrow linewidth laser and a method for obtaining a single-longitudinal mode ultra-narrow linewidth optical signal are provided in the present invention. The on-chip ultra-narrow linewidth laser includes a laser generating gain unit for generating a broad-spectrum initial optical signal and performing wavelength filtering on the generated optical signal, and also includes a distributed scattering feedback unit for performing linewidth compression on the optical signal; the laser generating gain unit is connected with the distributed scattering feedback unit, so that the optical signal generated by the laser generating gain unit is subjected to wavelength filtering and then output to the light guide component of the distributed scattering feedback unit to scatter to form an optical signal with a narrower linewidth to achieve linewidth compression, and the optical signal returning along the original path and fed back to the optical signal of the laser generating gain unit is subjected to gain amplification and wavelength filtering once again, repeating until achieving a steady state so as to obtain a single-longitudinal mode ultra-narrow linewidth optical signal. The laser can obtain a steady single-longitudinal mode ultra-narrow linewidth optical signal, and is simple in structure and small in volume.