The hub ratio of a bulb turbine has an important influence on the unit’s flow capacity, efficiency, and operating stability and is one of the key parameters governing the hydraulic performance and structural safety of the turbine. To select the optimal hub ratio matching the bulb turbine of a hydropower station after local thickening retrofitting of its runner blades, a full three-dimensional physical model of the unit was established, and numerical simulations were carried out to investigate the influence of hub ratio on turbine performance. Four hub-ratio schemes, namely 0.32, 0.34, 0.36, and 0.38, were selected for full flow passage simulations. The key hydraulic performance indicators of the four schemes were compared, revealing a nonlinear relationship between hub ratio and turbine performance: as the hub ratio increases, the overall hydraulic performance of the turbine first improves significantly and then declines continuously. The simulation results show that the turbine achieves the best overall performance when the hub ratio is 0.34. To verify the structural reliability of the hydraulically optimal scheme, stiffness and strength calculations of the runner were performed under typical operating and extreme conditions. The results indicate that, at a hub ratio of 0.34, both the stress distribution and deformation displacement of the runner are within the allowable limits specified by the design requirements, and the structural safety and reliability of the runner meet the operating requirements. This study provides a theoretical basis and engineering reference for parameter optimization, performance improvement, efficiency enhancement, and safe and stable operation of similar low-head tubular units.