To investigate the characteristics of air-extraction-induced negative-pressure penetration of bucket foundations and the effects of negative pressure level and bucket diameter on penetration behavior and pore-water pressure response, a series of 1g model tests were conducted using bucket diameters of 108, 127, and 141 mm under negative pressures of ?10, ?20, and ?30 kPa. A penetration resistance model was also established by considering seepage along the inner and outer bucket walls and the seepage-induced reduction in resistance at the bucket tip. The results show that, compared with the ?10 kPa condition, the penetration times under ?20 and ?30 kPa were reduced by 81.1%–87.4% and 87.7%–91.6%, respectively. However, when the negative pressure was further increased from ?20 to ?30 kPa, the penetration time decreased by only an additional 2.5–4.0 s, indicating a diminishing gain in penetration efficiency with further increases in negative pressure. The effect of bucket diameter weakened as the negative pressure increased, with the maximum difference in penetration time among the three bucket diameters decreasing from 36.5 s to 3.5 s and 2.0 s. The peak magnitude of negative excess pore-water pressure increased with increasing negative pressure, whereas the normalized pore-pressure response gradually decreased, and the peak values mainly occurred during the later stage of negative-pressure penetration. The theoretical model showed good agreement with the experimental results under relatively low negative pressure and during the near-quasi-static penetration stage, with R² values ranging from 0.875 to 0.954.