Abstract: To address the unclear horizontal bearing performance of novel pre-stressed high-strength concrete (PHC) enlarged-base piles in coastal mudflat soft clay areas and the inapplicability of existing theories, this study investigates the horizontal load response and deformation mechanism of enlarged-base piles. The research is based on full-scale field tests combined with three-dimensional numerical simulations utilizing the Modified Structured Cam-Clay (MSCC) model. The study reveals a unique "node anchoring" mechanism of the enlarged-base structure, demonstrating that the enlarged section alters the traditional distribution pattern of lateral soil resistance by establishing "high-resistance hard points" at specific depths. The results indicate that the radial expansion ratio (<italic>B/D</italic>) is the dominant factor in enhancing the bearing capacity, exhibiting significant spatial localization and an abrupt increase in resistance strength. Conversely, the axial expansion ratio (<italic>H/D</italic>) presents a vertically layered reinforcement effect, which mitigates the concentration of the bending moment along the pile shaft by extending the distribution of the high-stress zone. Given that the API standards significantly overestimate the initial stiffness and ultimate resistance of mudflat soft clays, this paper proposes a segmented modified <italic>p-y</italic> curve model tailored for enlarged-base piles. By introducing a stiffness enhancement coefficient (<italic>β</italic>_k) and a resistance enhancement coefficient (<italic>β</italic>_p) based on the geometric dimensions of the enlarged base, the model quantifies the nonlinear strengthening laws of the local pile-soil interaction governed by the expansion dimensions. Furthermore, considering the weakening effect of tidal water levels on the confinement capacity of the soil, a simplified calculation method incorporating a weakening reduction coefficient (<italic>β</italic>_F) is established. The modified model significantly reduces calculation errors, providing a reliable basis for the design and engineering application of photovoltaic enlarged-base piles in coastal mudflat environments.