Abstract: To maintain the vacuum degree inside a generator flywheel system, four types of shaft seals were designed. A numerical method based on thermo-fluid-solid coupling was employed to investigate the sealing leakage and internal heating characteristics of the flywheel system under different rotational speeds. The internal flow and temperature rise characteristics of the four schemes were compared. Results show that as the generator rotational speed increases, the leakage within the flywheel system decreases for all four schemes, with the minimum overall leakage occurring at the rated speed of 3 600 r/min. The incorporation of seal rings at the front and rear ends of the shaft seal significantly reduces leakage rate within the flywheel system, lowering the seal leakage by approximately 38% at the rated speed. Under the designed vacuum degree, increasing the number of seal teeth has a limited effect on the leakage rate of the flywheel system, with the leakage at rated speed reduced by only about 0.6%. As the generator rotational speed increases, the internal heat generation increases, and the temperature distribution unevenness of the rotor and casing intensifies. At the same rotational speed, the rotor and casing temperatures of the flywheel system under the four schemes are similar, with the maximum difference in the average rotor temperature at rated speed being 0.75%. Adding sealing rings at the front and rear ends of the shaft seal has little effect on solid heat generation. Among the four schemes, the sealing structure with a stepped single tooth and sealing rings at both ends achieves the lowest leakage and the lowest temperature rise for the generator flywheel system, effectively maintaining the vacuum degree inside the generator flywheel system.