Objective: The two-section gas centrifuge rotor connected via bellows is a technical approach aimed at enhancing the separative power of a single centrifuge unit. However, current numerical studies on this type of centrifuge design exhibit notable deficiencies. On the one hand, the cross-section of the bellows is often oversimplified as rectangular in computational models, leading to remarkable deviations in simulating gas flow near the bellows and reducing the accuracy of separative power calculations. On the other hand, boundary conditions in flow field simulations typically assume idealized sidewall temperature distributions—specifically, a linear temperature profile along the rotor sidewall—that substantially deviate from actual operational temperature distributions. In addition, existing models of the temperature field often oversimplify radiative heat transfer processes and overlook the coupled interactions between heat transfer and fluid dynamics within the rotor. To address these issues, this paper proposes two improvements to the numerical calculation method for the internal flow field of a two-section gas centrifuge rotor. Methods: First, a coupled-iterative numerical calculation method is developed to concurrently solve the flow field, temperature field, and radiative heat transfer. Leveraging the unique structural characteristics of gas centrifuges, the computational domain is divided into fluid field and solid regions, with coupled-iterative solutions achieved through continuity conditions at the regional boundaries. The fluid field region is solved using a predictor-corrector homotopy algorithm, an algorithm that is extensively applied and rigorously validated for its robustness and accuracy in modeling disturbed and strong swirling flows within gas centrifuge rotors. Convective heat transfer boundary conditions (of the third kind) are applied to simulate heat exchange among the outer casing of the gas centrifuge, the external environment, and cooling water. Radiative view factors between complex surfaces are calculated using integral methods to enable high-accuracy modeling of radiative heat transfer. This approach yields accurate flow and temperature field distributions under actual operating conditions and supports multiparameter optimization of the separative power of a gas centrifuge. Second, a staircase approximation method is introduced to simulate the complex cross-sectional shape of the bellows. This enhancement improves the accuracy of flow field simulations near the bellows and the accuracy of separative power calculations while enabling geometric optimization of the bellow design. Results: The application of the improved calculation method yields the following results: (1) the actual temperature distribution along the sidewall of the gas centrifuge rotor notably deviates from the ideal linear profile. Elevated temperatures at the product end generate reverse circulation, which adversely affects isotope separation efficiency. (2) Optimizing the emissivity of radiation heat transfer surfaces improves the temperature distribution along the rotor sidewall, thereby substantially enhancing separative power. (3) Incorporating staircase approximations for the below cross-section improves the accuracy of separative power calculations. (4) The staircase approximation facilitates shape optimization of the bellows, with a six-level approximation satisfying high-precision computational requirements. Conclusions: The improved numerical calculation method proposed in this paper for analyzing the flow field within the rotor of a two-section gas centrifuge enables a coupled solution of the flow field, temperature field, and radiative heat transfer under actual operating conditions. This approach markedly enhances computational accuracy for capturing flow dynamics near the bellows and for determining separative power. By leveraging this calculation method, multiparameter optimization of the centrifuge separative power, as well as shape optimization of the bellow cross-section, can be effectively achieved.