Abstract: Tight gas reservoirs such as terrestrial sandstone and marine carbonate rock are characterized by strong heterogeneity, complex gas-water relationship, and low porosity. Quantitative evaluation of gas-bearing properties is an inevitable requirement for formation evaluation. Due to the hydrogen index and the fast neutron interaction process of gas, water and oil is different obviously, fast neutron cross-section (FNXS) has become a quantitative method for evaluating gas saturation. This paper uses Monte Carlo software to establish borehole and formation models under different porosity and shale content conditions, simulates the distribution of inelastic scattering gamma counts, and studies the relationship between inelastic scattering gamma counts, FNXS and gas saturation. The results show that FNXS can be characterized by inelastic gamma counts, and the shale content have influences on FNXS characterization and gas saturation response, the correction for shale are needed. The results of field examples show that FNXS can realize quantitative evaluation of gas saturation, and the interpretation results are consistent with open hole results. The research provides technical support for the quantitative evaluation of tight gas reservoirs.