Abstract: Under the vision of “dual-carbon”, CCUS (carbon capture, utilization and storage) technology has become a key means to reduce CO₂ emission, ensure energy security and realize sustainable development in China, while quantitatively evaluating CO₂ content is an urgent problem in CCUS technology. This paper, based on the theory of neutron diffusion in the formation, analyzes methods for quantitatively assessing CO₂ using neutron reaction cross-sections and deceleration length. Using the Monte Carlo method, a computational model was established to simulate the interaction of fast neutrons with CO₂ and CH₄-bearing stratigraphic media, and to study the variation rules of parameters such as scattering cross section, macroscopic capture cross section, and deceleration length of fast neutrons and the saturation degree of CO₂ and CH₄, in order to screen the neutron logging methodology of quantitative evaluation of CO₂. The simulation results show that under certain porosity conditions, the deceleration length is more obvious than the fast neutron scattering cross section and macroscopic capture cross section in recognizing the dynamic changes of CO₂ and oil and water, and it can distinguish between CH₄ and CO₂. These findings provide a foundation for designing instruments and data processing methods for quantitative CO₂ evaluation using D-T neutron sources and multi-detector logging systems, which is of significant importance for the development of CCUS technology.