Two large-scale sinking tests of the hybrid bucket foundation were revealed in typical saturated silty clay of China offshore wind farms. The experimental foundation has an outer diameter of 3.5m and a clear wall height of 0.9m with seven rooms divided inside the HBF by steel bulkheads, which are arranged in a honeycomb structure. The different pressure inside the seven compartments can control the levelness of the HBF during suction installation. During the sinking process of the bucket foundation, real-time monitoring was conducted of the produced underpressure, the soil pressure and the pore pressure inside and outside the bucket wall, which reflected the sinking state of the bucket foundation. Test results show that suction can be combined with air pumping to reasonably control the sinking speed of the bucket foundation and the levelness at each stage. In the cases where the soil surface at the sinking location for the bucket foundation was relatively horizontal, the levelness could be ensured to meet the standard and design requirements after the sinking of the bucket foundation was completed. •The penetration testing of the composite bucket foundation with 7 compartments.•The sinking technique of sinking–adjusting–stabilizing–sinking process.•The tilt adjusting technique by applying suction/positive pressure and intermittent pumping among the 7 compartments.