Abstract: The Monte-Carlo (MC) code MCNPX was used for the calculation of the correction factors caused by the chamber geometric structure, the ionized gas and air when a 238U fission chamber is used to determine the neutron fluence rate of 14.8 MeV and 25.5 MeV quasi-monoenergetic neutrons produced by 5SDH-2 and HI-13 tandem accelerators, the cosmic neutron source and the spallation neutron source. The detection responses for these neutrons were also given. Because the sensitive layer of the detector is too thin, the Dxtran spheres and force collisions were combined to improve the calculation efficiency and to reduce the variance. In the case of the energy range of the source spectrum is wide and high or low energy particles are infrequent such as the cosmic neutron source or the spallation neutron source, the higher energy part and the lower energy part were calculated separately to improve the calculation efficiency. Finally, the correction factors were used for the calculation of the detection response with formula, and the detection responses calculated with formula and by MC simulation were compared with each other. The detection response of the 238U fission chamber for 14.8 MeV neutrons was also calibrated by the associated particle method. The measured result was compared with the calculated one from MC simulation.