Abstract: A detector of ⁶LiF semiconductor sandwich spectrometer was designed and manufactured. Characteristics of the spectrometer were tested in the fast neutron critical assembly. Measurement principle, configuration of detector and electronic circuit were introduced. Fast neutron spectrum was measured using the ⁶LiF semiconductor sandwich spectrometer. When the detector’s ⁶LiF mass thickness is 186 μg/cm², the spectrometer’s energy resolution is 363 keV in the thermal neutron field. For this spectrometer, the optimal fathomable neutron energy range is 0.3-7.5 MeV, and the background counts only take possession of 1% in this area.