Abstract: As a new type of offshore wind power foundation, the composite bucket foundation has large difference in structural form from the traditional bucket foundation, thus its ultimate bearing capacity and bucket-soil interaction mechanism need further research. Aiming at this problem, a large-size model for the composite bucket foundation is designed, and several horizontal bearing capacity tests are carried out in clay. The distribution of soil pressure, the motion mechanism and the ultimate bearing capacity of the composite bucket foundation are determined depending on the tests. The test results show that the soil pressure on the passive zone increases significantly with the increase of horizontal load, while the soil pressure on the active zone has negative growth at first and returns to the initial value finally. The position of the instantaneous rotation center moves from the central axis to the bottom of the bulkhead at the loading direction with the increase of horizontal load, and it’s approximately 0.8L below the ground surface around the bulkhead at the loading direction under the ultimate horizontal load.