Abstract: The absolute neutron fluence can be determined by the ²³⁸U(n,f) cross-section and the number of the fission fragments. But this method can’t be used in the neutron energy ranges of E_n＜1.6 MeV and 6.0 MeV＜E_n＜7.0 MeV. Using four nuclear reactions as neutron source, the relative efficiency of BF3 long counter in the neutron hall of 4.5 MV Van de Graaf accelerator at Peking University was calibrated. The absolute neutron fluence in the above mentioned energy ranges can be determined by the combination of the ²³⁸U(n,f) reaction and the calibrated BF₃ long counter.