Large-diameter rock-socketed piles have been extensively used as the foundation of platforms and other offshore installations. In this paper, the behavior of large-diameter rock-socketed CFST (concrete-filled steel tube) piles under lateral loads is studied, based on field tests and numerical analysis. The CFST piles are 2800 mm in diameter and 40 m in length, with 5.2 m socketed into the rock. The horizontal capacity and deformation of large-diameter rock-socketed piles are analyzed from the measured displacements and internal forces of piles. The interactive behavior of pile-rock and the influence of backfilled sand on horizontal capacity are also discussed. Using the Finite Element Method (FEM) considering the properties of the pile-soil interface, the test results are simulated numerically. This shows that the stress concentration effect in the region near the bottom of the steel tube should be considered in the design, because the socketed part of piles bears most of the lateral load. The steel tube and backfilled sand are both effective in limiting the pile displacement under the working load. Parametric analysis shows there is an optional socketed length to improve the bearing capacity of piles.