Due to the motion of light source, one well known difficulty in fast-beam spectroscopy is the large line width caused by the Doppler shift of light across the collecting optics when a spectrometer is coupled to the beam in a conventional manner. In order to study the high-resolution beam foil spectroscopy of an 800-keV He+-ion beam passing through a transverse magnetic field, an off-axis Fabry-Perot interferometer was used and a line width of about 0.2 A was observed. The principle of coupling a fast beam to the off-axis Fabry-Perot to obtain Doppler compasation is described along with experimental results.