Abstract: In order to make further studies on fusion neutron diagnosis on HL-2A /HL-2M, we have developed and succeeded in the calculation of the Response Function for a Bonner sphere spectrometer, which consists of eight polyethylene spheres with ³ He proportional counters inside. The response function of the Bonner spectrometer to neutrons is of fundamental importance for its neutron spectrum unfolding procedure and is directly related to the quality of the unfolded spectrum. In this paper, we calculated the response function to neutrons from 10 ⁻⁹ MeV to 100 MeV by Geant4. In order to test the accuracy of the Geant4 simulation, we apply it to measure an ²⁴¹ Am-Be neutron source, and the measured neutron counts of the spectrometer and simulated counts are found to be highly consistent, with a relative error up to 9.3%. This has proven the calculation of the neutron response of the Bonner sphere spectrometer by Geant4 to be quite accurate.