Abstract: Owing to the radiation safety risks posed by radioactive sources, the technical approach of substituting D-T neutron generators for ¹³⁷Cs gamma sources in formation density measurement has garnered growing attention within the industry. Based on the neutron-gamma coupled field theory, this paper derives the flux distribution characteristics of inelastic scattering gamma rays and epithermal neutrons. Via Monte Carlo numerical simulations, it is revealed that as formation porosity increases and density decreases, the epithermal neutron count exhibits a rapid declining trend with a narrowed distribution interval, whereas the inelastic scattering gamma count decreases at a relatively slow rate with an expanded distribution range. On this basis, this paper further investigates the intrinsic correlations among inelastic scattering gamma count, epithermal neutron count, formation density, and hydrogen index, and establishes a dual-detector formation density calculation method that integrates a D-T neutron source with epithermal neutron detectors and gamma ray detectors. The research demonstrates that when the source-detector distances of the epithermal neutron detector and gamma ray detector are set at 30 cm and 75 cm, respectively, the measurement accuracy of this method for formation density can reach 0.021 g/cm³. The results of simulation experiments validate the feasibility and effectiveness of the combined detection technique using epithermal neutrons and inelastic scattering gamma rays for formation density determination.