Abstract: Quantitative study of micropaleontology is carried out for the core LYK drilled at the northern part of the abandoned Yellow River delta. 106 benthic foraminiferal species of 68 genera and 46 ostracod species of 30 genera are discovered. By Q-mode hierarchical cluster analysis, the foraminifera found can be divided into five assemblages. AMS¹⁴C dating and grain size data are adopted to reconstruct the environmental evolution over the past millennia. The results suggest that the study area was dominated by fine-grained and well-sorted shelf sediments and the microbiota assemblage was close to normal marine facies before the Yellow River poured into the sea. The study area started receiving enormous coarse-grained sediments with worse sorting after the Yellow River seized the Huaihe river channel and entered the sea. There appeared a large number of foraminifers and ostracods of nearshore brackish water genus, and the sedimentary environment was changed from a typical shallow sea to a delta-front. Once the Yellow River moved to the north, large sediment supply was cut off. Under the joint action of waves and tidal currents, the study area was suffered from erosion and redeposition, and the sand/mud ratio was thus increased. The abundance and diversity of microbiota declined sharply, while the wide salinity microbiota increased. With the gradual southward movement of the erosional center, the dynamic conditions of the study area became stabilized. The erosional depressions formed in the early stage gradually accepted sediments from both the runoff and tidal current, and a large number of floating foraminifers and off-site molecules appeared. In recent years, along with the reworking of waves and tides, the study area gradually becomes flatter and changes from a turbulent shoreline to a stable shallow sea.