<p>Large complex thin-walled ring-in-ring castings typically contain multiple I-shaped structures comprising an inner ring, an outer ring, and supporting plates. During solidification, hot spots tend to form at the joints of these I-shaped structures, often leading to shrinkage porosity and cavity defects. To address this challenge, a relatively complex local gating system is required to provide targeted compensation. However, the traditional modulus method, which is commonly employed in gating system design, relies heavily on empirical knowledge and fails to fully consider the influence of casting geometry and material thermal properties on solidification behavior. Therefore, even with simulation verification, multiple design iterations are required, which result in high design costs and low efficiency. To address these limitations, this study proposes a design method based on the feature number for the feeding system of local I-shaped structures in castings, which involves the systematic calculation of feature numbers to guide feeding system design. The simulation results demonstrate that compared with the traditional modulus method, the proposed method effectively reduces reliance on empirical knowledge, achieves a superior reduction in casting defects, and reduces the design time to 1/3 of that required by the traditional method. Moreover, the casting process yield increases from 41.3 to 48.8%. This study provides a quantitative and efficient approach for the precise design of gating systems in large complex thin-walled ring-in-ring castings.</p>