Large-scale thin-walled variable-section ZTA15 titanium alloy castings hold significant application potential in the manufacture of complex, variable-section nozzle components for aeroengines. However, the casting process is highly sensitive to wall thickness parameters, leading to issues such as uneven mold filling, imbalanced temperature distributions, and solidification defects. ProCAST software was employed to conduct numerical simulations of the pouring and filling process, temperature field, velocity field, and solidification behavior of ZTA15 titanium alloy castings with different wall thicknesses (i.e., 2.0, 3.5, and 8.0 mm). The results reveal that the 2.0 mm thick-walled castings exhibit excessive filling velocity, resulting in vortex and porosity defects. The 8.0 mm thick-walled castings experience prolonged solidification time, increasing the risk of shrinkage cavities. In contrast, the 3.5 mm wall thickness yields a uniform temperature field and areasonable solidification sequence, with initial cooling occurring at the cross-shaped runner and the first layer of reinforcing ribs, followed by accelerated cooling of the large flat thin-walled surface in the later stages. Shrinkage porosity and cavities are observed across all wall thicknesses, primarily concentrated at the junction of the central sprue and the cross-shaped runner, as well as in the thicker regions near the reinforcing ribs. These defects are attributed to localized variations in cooling rates, leading to thermal stress concentration. No significant isolated liquid phase regions are found on the large flat thin-walled surfaces, indicating that optimizing the pouring velocity and time can effectively mitigate defects.