A numerical method was used to study the radial turbine of a 0.6 L two-stroke gasoline engine turbocharger, and it was found that different turbine disc structures have a significant impact on the leakage flow through the backface clearance. The backface clearance leakage flow formed a large-scale channel vortex after entering the main flow channel, which reduced turbine efficiency. Among the three types of structures, the deeply scalloped turbine had the greatest impact, therefore resulting in the lowest turbine efficiency. In contrast, the scalloped and unscalloped disc structures could attenuate or even eliminate the backface clearance leakage flow, increasing the turbine efficiency by 1.1% to 1.6% and 3.2% to 3.5%, respectively. The backface clearance leakage flow generated high excitation forces in the leading edge root area of radial turbine blades. All different disc structures had such high excitation zones in this area, although the range decreased successively. The comparative analysis showed that the deeply scalloped turbine experienced significantly higher generalized pressure due to the partial coincidence of high excitation zones and high modal amplitude regions at the blade leading edge. However, the high excitation force zones at the leading edge of the scalloped and unscalloped turbine did not overlap with high modal amplitudes, resulting in maximum generalized pressure reductions of 74% and 78%, respectively. In addition, combined with the modal analysis of different turbine disc structures, the first-order frequencies of scalloped and unscalloped turbines increased by 44.5% and 51.1% compared to the deeply scalloped turbine, respectively. These high frequencies made resonance conditions less likely to occur under operational loading, thereby enhancing structural reliability. Accordingly, the scalloped disc structure achieved an optimal compromise between minimizing turbine efficiency losses caused by backface clearance leakage and mitigating the risk of high-cycle fatigue failure of the blades.