An onboard system was built to simulate ship impact on the pile group foundation of offshore wind turbines. Four centrifugal model tests on a 2×2 pile group were conducted in saturated dense sand, including the tests on pile groups with and without an upper wind turbine subjected to lateral impact or eccentric impact with an eccentricity of 4.5 times pile diameters. The influence of the exist of wind turbine on top of pile cap and the impacted position by ship on pile cap on the impact force and the dynamic response of pile groups was investigated. Data from the model tests showed that strong dynamic interaction existed between the wind turbine and its underlying pile group. The motion of the pile cap made the wind turbine cause both lateral and torsional vibrations. The wind turbine shared and dissipated part of the energy transferred from the ship, which made the acceleration amplitudes of the pile cap attenuate quickly in the free vibration phase. In addition, the impact position on pile cap significantly affected the impact force and the internal forces of the group piles. The measured peak impact force from the test on the pile group without wind turbine subjected to eccentric impact was 20% smaller than that from the test on the pile group subjected to lateral impact, and the impact velocity in the former test was 26% higher than that in the latter. The maximum ratio of the peak pile-head bending moments of the piles was 3.2 in the test on the pile group without wind turbine subjected to eccentric impact. Generally, comparing with lateral impact on pile cap by ship, eccentric impact is more critical to make a pile group fail.