The invention discloses a neutron gamma density logging method by utilizing a distribution symptom of a fast neutron field. Fast neutron scattering and a gamma attenuation theory are used to represent distribution of an inelastic scattering gamma field, and mathematical relations between the gamma field distribution and parameters including the density, the free path of fast neutron scattering and a cross section of inelastic scattering are provided; on the basis of distribution of the inelastic scattering gamma field, a method for calculating the density via the free path of fast neutron scattering and inelastic scattering gammas of two detectors is obtained; the measured amount of fast neutrons is used to represent the free path of fast neutron scattering, and the new method for calculating the stratum density directly via the inelastic scattering gammas and the fast neutron field is obtained. The method avoids correction of hydrogen contained indexes, and provides technical support and theoretical guidance for gamma density logging.