Abstract: The medium-type fast-neutron detector responds to direct neutron irradiation and it has an extreme effect on its design and practical applications. Monte-Carlo simulation programs based on Geant4 toolkit are developed to study the neutron direct radiation response sensitivity of the medium-type fast-neutron detector. The emission of the charged heavy particles of commonly used materials of detector’s sensitive unit due to neutron incidence is simulated. The result shows that the selection of the materials of the charge collector, the threshold absorber and the rear isolator directly affect on the response sensitivity of neutron direct radiation. The response sensitivity of neutron direct radiation contributes about 6% to the total neutron sensitivity in the present sensitive unit which us