To obtain sediment samples beneath the Antarctic Ice Sheet with a thickness of 3000~4000 m, the diameter of corer is strictly limited (≤200 mm), and subglacial sediments may exhibit relatively high strength. The vibratory coring method presents advantages for this scenario, yet existing open-water vibratory corers struggle to maintain the required penetration capacity when their diameter is reduced. In this paper, a vibrator is designed to achieve sufficient penetration capacity under dimensional constraint. Adopting the configuration of “vertical motor + timing-belt transmission + axially arranged eccentric blocks”, this vibrator can superimpose excitation forces of each eccentric block under a diameter of 160 mm, with a theoretical designed excitation force of 10.08 kN at 50 Hz. Laboratory tests on the three- and four-eccentric-block prototype demonstrate that this scheme performs well mechanically. The four-block assembly with pressure housing also vibrates normally, and no structural abnormalities are observed in laboratory tests. It can be inferred from the test results that, with further improvements based on the current prototype, the corer is expected to meet the theoretical design targets. The six-block configuration and actual penetration performance remain to be verified in future work.