Abstract: The composite foundation with plain concrete piles is applied in the construction of a high-rise building. Various in-situ loading tests and stress tests of piles and soil are performed after excavation of foundation pits. The settlement and stress variations of the composite foundation are monitored during the construction period. Based on the test data, the work mechanism, load sharing characteristics of composite foundation, stress distribution of the piles, and settlement characteristics of the foundation are analyzed. Moreover, the effects of post-grouting technology of the piles on the work performances of the composite foundation are evaluated. The test results indicate that the composite foundation with plain concrete piles exhibits high bearing and small deformation capacities. The reinforcement without soil compaction technology has a reinforcing effect on the soil among piles. The application of post-grouting technology can improve the bearing capacity and reduce the settlement of the composite foundation, and high loading level may result in better reinforcing effects. The rigidities of the foundation slab and upper structure provide certain adjustment for non-uniform loads on piles. As a result, a variable rigidity design method based on the distribution of load and foundation pressure is proposed.