Hydrogen-oxygen cycle induced steam thermal loads drive critical heat transfer reevaluation in hydrogen turbine disk cavity. This study investigates the flow characteristics and their impact on wall heat transfer coefficients within high-pressure turbine rotating disk cavity using water vapor as the working medium, specifically addressing the secondary air system in high-temperature turbine sections of pure hydrogen-oxygen cycles. Considering the unique combustion products in hydrogen-fueled gas turbines, systematic analysis reveals that the wall heat transfer coefficients exhibit strong correlation with fluid velocity gradients and vortex structures. Comparative analysis between 1D fluid network methodology and 3D CFD simulations demonstrates that 1D predictions yield conservative yet engineering viable results, with a maximum deviation of 41.4% observed at the rotor wall. The 3.11% temperature field prediction discrepancy remains within acceptable thresholds for rapid thermal assessments in turbine design.