There has been a growing interest in the application of a controllable neutron source for the purpose of measuring formation porosity in the advancement of nuclear logging. The existing porosity algorithm, which is based on the thermal neutron counting rate, shows a lower sensitivity in the high porosity region. To enhance the sensitivity, the effects of elastic and inelastic scattering, which influence the slowing-down of fast neutrons are theoretically analyzed, and a slowing-down model of fast neutrons is created. Based on this model, a density correction porosity algorithm is proposed according to relationship between density, thermal neutron counting ratio and porosity. Finally, the Super Multi-functional Calculation Program for Nuclear Design and Safety Evaluation (TopMC/SuperMC), was used to create a simulation model for porosity logging and the applicability was examined. The result shows that the relative error between the calculated porosity and the actual porosity is less than 1%, and the influence of deviation in the density measurement is less than 2%. Therefore, the proposed density correction algorithm based on the slowing-down model of fast neutrons can effectively improve the sensitivity in the high porosity region. This study has certain reference significance for the application of neutron porosity measurement with D-T neutron sources