Abstract: Cretaceous phytoplankton from the newly completed core hole (SK-1, N&S) in the central Songliao Basin was studied. The target interval is from the upper Quantou Formation of Upper Cretaceous to Mingshui Formation. Twelve genera of dinoflagellates, six of acritarcha and three of chlorophyta were identified in 588 samples from the 2300-m deep core. Ten phytoplankton assemblages have been classified: Tetranguladinium-Subtilisphaera-Botryococcus, Botryococcus-Pediastrum, Pediastrum, Dinogymniopsis-Chlamydophrella-Vesperopsis bifurcate, Dinogymniopsis minor-Balmula, Pediastrum-Botryococcus, Schizosporis-Campenia, Kiokansium-Dinogymniopsis-Botryococcus, Dinogymniopsis-Granodiscus-Filisphaeridium and Granodiscus. According to the findings mentioned above, the age of the upper Qantou Formation is Cenomanian, the Qingshankou Formation belongs to upper Cenomanian-Turonian, the Yaojia Formation to Coniancian-Santonian age, the Nenjiang Formation is of Campanian age, and Sifangtai-Mingshui formations are referred to Maastrichtian stage. The ecology of phytoplankton is closely related to water salinity. Each type of phytoplankton is within a certain living water mass whose evolution type reflects salinity change of the Songliao Lake. The assemblages from the SK-1 indicate that water salinity changes with the cycle of freshwater–slight brackish water–brackish water–slight brackish water–freshwater in the lake.