Abstract: The fast neutron detector using boron-lined ionization chamber was deve loped. The Bonner multi-cylinder neutron detector consists of a 40 mm×200 mm boron-lined ionization chamber clading with high density polyethylene cylinder, which was used as fast neutron moderate and with external diameters of 55, 80, 130, 220 and 285 mm. The energy range of neutrons is broad, extending from thermal neutron to tens MeV fast neutron. The response function of Bonner multi-cylinder neutron detector was simulated by Geant4 code. The experiments based on the standard neutron source were done. While the outer diameter of high density polyethylene is 220 mm, the neutron sensitivity of Bonner multi-cylinder neutron detector reaches a level of 8.702×10^-15 A•cm²•s under experimental conditions. The experimental data were compared with the simulation results. It is shown that the simulation results are accordant with experiment results in observation data error.