In high-fill loess sites, extra-long piles commonly penetrate a two-layer foundation composed of upper artificial fill and underlying natural loess. The asynchronous mobilization of shaft resistance, complex load-transfer path, and insufficient basis for bearing-capacity evaluation make their bearing behavior difficult to predict accurately. In this study, an analytical load-transfer model for extra-long piles is developed by considering staged hardening of the pile–soil interface. The shaft load-transfer relationship is divided into three stages: elastic response, plastic hardening, and ultimate sliding. Differences in interface stiffness, critical displacement, and ultimate shaft resistance between the upper fill and the lower natural loess are incorporated, and analytical expressions for pile displacement, axial force, and shaft resistance are derived for different loading stages. The model is validated using two field static load-test cases. The results show that the calculated responses agree well with the measured data, with the mean absolute percentage error (MAPE) below 13% over the entire loading process. The proposed model can reasonably capture the gradual load–settlement response of extra-long piles in loess, as well as the progressive mobilization of shaft resistance from shallow to deep soil layers and the segmented attenuation of axial force along the pile shaft. Parametric analyses indicate that increasing pile length improves the ultimate bearing capacity, but the marginal contribution of additional pile length decreases progressively. When the weak-fill thickness ratio β increases from 0.125 to 0.625, the ultimate bearing capacity decreases by approximately 45%; when the hardening stiffness ratio α increases from 0 to 1.0, the ultimate bearing capacity increases by approximately 75%. These findings indicate that the bearing capacity of extra-long piles in high-fill loess is jointly governed by staged hardening of the pile–soil interface and the thickness of the overlying weak fill layer. The proposed model provides a useful reference for bearing-capacity calculation and design-parameter selection for extra-long piles in high-fill loess foundations.