<p>Fast neutron multiplicity measurements represent a powerful tool for the non-destructive analysis of nuclear safeguards and they are based on the point model assumption. The effect of spatial neutron multiplication was studied with the Geant4 code by conducting a comprehensive analysis of the non-adaptability of the point model hypothesis. Our work shows that the distribution of induced fission events and the neutron flux density exhibit non-uniform changed across the sample’s volume with a relevant correlation between the incident neutron energy and the average number of neutrons released from a single fission event. Based on the above analysis, there is a deviation between the multiplication coefficient solved by the point model equation and the actual value, which will finally lead to deviation of nuclear material quality. A correction method based on the multiplication coefficient is proposed, which effectively reduces the measurement deviation.</p>