The invention discloses a method for measuring a neutron energy spectrum of a neutron radiation field by utilizing a lanthanum bromide detector. The method comprises the following steps: (1), establishing energy distribution function relationship (as shown in the description), generated by inelastic scattering of neutrons and a detector, of a neutron characteristic gamma energy peak net count andneutron energy, wherein phi(Ei) is neutron fluence of an ith energy group of a to-be-tested neutron energy spectrum; Nngamma is a net count, generated by inelastic scattering of the neutrons and <79>Br, <81>Br and <139>La, of a jth characteristic gamma energy peak Engamma; epsilongamma is intrinsic full-energy peak detection efficiency, generated by the detector to the inner part, of thejth characteristic gamma energy peak Engamma; sigmaj(Ei) is a reaction cross section, generated by a reaction of neutrons with energy as Ei and kth nuclide, of the jth characteristic gamma energy peak; Nj is nuclear number for the kth nuclide and the neutrons to generate a jth reaction in a crystal; and Pji(Ei) is probability that the ith energy group neutrons participate in the jth reaction; (2), establishing j kinds of response equations for obtained j kinds of neutron characteristic gamma energy peak and forming an equation set; (3), determining response functions of the neutron characteristic gamma energy peak to different energy neutrons Ei; and (4), analyzing the equation set.