Abstract: The neutron scattering model is one of the key models required for solving the neutron transport equation, and it significantly influences the processing of scattering cross-sections and the final results of neutron transport calculations. Within the traditional theoretical framework, in the thermal energy region, a free gas scattering model was used for the most of nuclides, assuming that the scattering cross-section remains constant with energy. For other minority nuclides, such as hydrogen in water and carbon in graphite, a more complex S(\alpha ,\beta ) model accounting for chemical binding effects was utilized. In the resonance energy region, a more simplified asymptotic scattering model was adopted. The asymptotic scattering model only has down-scattering, which is inconsistent with the actual situation where neutrons may gain energy due to the thermal motion of nuclides, leading to up-scattering. When free gas scattering models were first introduced into continuous-energy Monte Carlo in the 1950s, the constant scattering cross section approximations in physics modeling were considered acceptable. Due to computer limitations, statistical uncertainties were of that size or larger. Until 1991, Mohamed Ouisloumen and Richard Sanchez published a paper pointing out that due to the strong scattering resonance of heavy nuclei (such as ²³⁸U) in the resonance energy region, using asymptotic scattering model only considering down-scattering would underestimate resonance absorption, resulting in higher reactivity calculation results. A more accurate free gas model considering resonance up-scattering effects is needed to improve computational accuracy. Since then, the issues of scattering cross-section resonance effect and up-scattering effect in the resonance energy region have been given renewed attention. To improve computational accuracy, the scattering model of the resonance energy region has been improved to a more accurate free gas model and the assumption of constant scattering cross-section has been eliminated for advanced Monte Carlo and deterministic codes, such as MCNP6, SERPENT, PARAGON2, CASMO5, etc. The free gas model assumes that the velocity of neutrons follows the Maxwell distribution after colliding with nuclides, which can well consider the influence of nuclide thermal motion on neutron elastic collision. The combined effects of up-scattering, scattering and absorption cross section resonance will have a complex impact on resonance absorption of neutron, ultimately affecting the results of reactivity and Doppler temperature effect. The processing method of resonance up-scattering effect based on global-local coupling with ultra-fine group resonance calculation method was researched in this paper, in order to further improve the calculation accuracy for the reactivity and Doppler temperature effect of pressurized water reactor problems. This method applied a free gas scattering model to accurately handle the resonance up-scattering effect of the main resonance nuclides by solving the ultra-fine group slowing down equation iteratively one by one for each resonance peak. While retaining the high accuracy, applicability, and efficiency advantages of the original resonance calculation method, this method has the advantages of easy implementation and expansion to other resonance nuclides. For the Doppler reactivity feedback benchmark, the improvement in Doppler temperature coefficient calculation accuracy achieved by this method falls within the range of −0.46%, −0.03% compared to the CASMO5. Additionally, for the typical pressurized water reactor cell problem, this method improves the accuracy of reactivitg calculations by approximately 200 pcm, which also consistent with the results of the CASMO5. The numerical calculation results indicate that this method can accurately process the resonance scattering effect, thereby effectively improving the calculation accuracy of the reactivity, Doppler temperature effect and feedback of pressurized water reactor problems. Simultaneously, the analysis of the influence of free gas scattering model on the fine energy spectrum and absorption reaction rate of pressurized water reactor cell problem demonstrates the mechanism of reactivity changes caused by resonance scattering effect.