Abstract: This article is the result of geo-environmental survey engineering.Objective Microfossils, especially pollen, spores, and dinoflagellate cysts, as direct indicators or proxies, provide significant information on geological environment. Therefore, palynological analysis has an important role in restoration and reconstruction of vegetation history and sea-level fluctuations, which provides information from the sediment cores, especially in alluvial coastal plain regions.Methods In this study, we carry out analyses using detailed palynological records of pollen, spores, freshwater algae, marine dinoflagellate cysts, and foraminiferal organic linings from Borehole HLL01 on the southern coastal plain of Laizhou Bay, Bohai Sea, northern China.Results The results provide valuable information on vegetation history and sea-level fluctuations since the Late Pliocene. Large amounts of terrestrial herb pollen (75.13%; mainly composed of Chenopodiaceae, Artemisia and Poaceae) were identified at Borehole HLL01, suggesting that coastal steppes flourished in the study area and its adjacent areas, and a cold and dry climate than the present during the Late Pliocene to Early Pleistocene, ca. 6.8-1.6 Ma. After that, the steppe areas began to shrink, and the transition of coastal steppes to forest grasslands occurred in the Early-Middle Pleistocene, ca. 1.6-0.6 Ma, indicating a warm and humid climate. A sharp increase in freshwater algae indicates that large rivers' discharge had been injected into the study area, or large lakes were formed. Afterward, relatively cool and dry climatic conditions appeared, with little precipitation, and a relatively significant transgression event revealed by dinoflagellate cysts and foraminifera organic linings, and forest grassland vegetation was distributed in the study area and its adjacent areas, during the Middle Pleistocene to Holocene, ca. 0.6-0.004 Ma.Conclusion Three ancient plants have occurred and the sea level fluctuations occurred since the Late Pliocene(6.8~0.004 Ma) in the Laizhou Bay,