Large-sized, thin-walled ZTA15 titanium alloy castings with variable cross-sections have broad application prospects for complex aeroengine nozzle components. However, owing to variations in wall thickness, the casting process faces challenges such as incomplete filling, nonuniform temperature distribution, and solidification defects. ProCAST software was employed to perform numerical simulations to optimize the casting process of the thin-walled ZTA15 components. The effects of different gating system designs on the temperature field, flow field, solidification behavior, and microstructural evolution were systematically analysed. The results reveal that conventional central and radial cross-runner systems tend to generate high-temperature hot spots, leading to uneven solidification and a concentration of shrinkage cavities and porosity defects. In contrast, the optimized-side auxiliary runner system effectively improves the uniformity of the temperature and flow fields through flow diversion and feeding, reduces molten metal impact, and minimizes gas entrapment and slag inclusion. Consequently, shrinkage and porosity defects are redirected into the gating system. The simulated microstructures indicate that the optimized design promotes a more homogeneous microstructure.