In this paper, we propose and demonstrate a single-element beam shaper for transforming a fiber laser beam into a near-diffraction-limited dark hollow beam. The single-element beam shaper contains two aspheric surfaces. One aspheric surface redistributes the intensity distribution of the incident beam and the other re-collimates the output beam. The distributions of these surfaces are derived by the energy conservation condition and constant optical path length condition. The comparisons between the single-element beam shapers based on different working principles are analyzed in detail. Based on the Fourier optics and Geometrical optics, the near field, far field intensity distribution and wavefront distribution of the output beam are studied in detail. The influences of deviations of the beam shape from the assumed value, distance between dual aspheric surfaces and optical alignment errors are studied in detail. Results show that the shaping errors of the single-element Keplerian beam shaping system are much smaller than that of the single-element Galilean beam shaping system. The wavefront distribution of the output beam is maintained. The dark hollow intensity distribution of the output beam can be maintained for a certain distance in the near field and the far filed intensity distribution exhibits airy disk pattern.