<p>Large-diameter composite piles (LDCPs)—a new deep foundation that couples a precast pile core, grouted soil, and an outer cement mixing column—are gaining attention for highway bridges, yet no rational model exists for their axial capacity. To fill this research gap, this study conducts an investigation into the vertical bearing characteristics of large-diameter composite piles through centrifugal model tests and theoretical calculations. Based on the radial heterogeneous dielectric distribution of the pile, the equivalent elastic modulus is introduced, and a bearing capacity calculation method is proposed by utilizing the hyperbolic load transfer model. The results indicate that the theoretical values are in good accordance with the measured data from centrifugal model tests. However, the discrepancy between them widens as the pile length and diameter increase, which can be attributed to the simplified selection of the minimum pile side friction in theoretical calculations. Further analysis reveals that both the pile diameter and length have a positive influence on the vertical ultimate bearing capacity. Specifically, the bearing capacity increases linearly with the pile length, and the increase is more pronounced when the pile diameter is enlarged. Additionally, an increase in pile length enhances the skin friction while reducing the tip resistance, whereas an expansion of the pile diameter mainly improves the tip resistance with minimal impact on the skin friction. This research offers valuable theoretical and experimental references for the design and application of large-diameter composite piles in engineering projects.</p>