Abstract: The new method for thermal neutron flux density measurement by γ spectrometer is described. The neutron flux density corresponding to 411 keV γ ray of 152 Eu is obtained according to the 411 keV γ ray peak area ratio of 152 Eu and 198 Au irradiated in reference field and the absolute neutron flux density measurement in reference field obtained by gold (Au) foil activation method. The gold foil is irradiated in the unknown neutron field. The 411 keV γ ray peak area for 198 Au irradiated in unknown field and 152 Eu irradiated in reference field can be obtained by other γ spectrometer. The absolute neutron flux density in the unknown neutron field can be gotten by the peak area ratio and the neutron flux density obtained corresponding to 411 keV γ ray of 152 Eu.