In recent years,dinoflagellate blooms had occurred frequently, and most of them were caused by the species of genus Peridinium and Peridiniopsis, and caused by the species of genus Ceratium were rare. Recently, however, the genus Ceratium species had been found throughout the world in the water bodies such as eutrophic reservoirs and fish ponds. In June 2019, some samples of the genus Ceratium were collected at various sites in Xiashan Reservoir of Shandong Province and found that it formed obvious light brown water blooms at local sites. Phylogenetic analyses showed that this species had the closest relationship with the common species C. hirundinella of the genus Ceratium, but the morphologically, the former had one intercalary plate and three apical plates, which were different from the C. hirundinella with four apical plates. By contrast, it is a newly recorded species of dinoflagellates-C. furcoides, which do not have been described in China. It has been reported as an invasive and water blooming species, which is often found in eutrophic water bodies during high-temperature seasons. In the Xiashan Reservoir, the chlorophyll-a concentration at the bloomed site is as high as 125.77 μg/L, and the density of the C. furcoides is 1.23×10⁶ cells/L, accounting for 1.21% of the total algal cell density. The number of cells is dominated by some filamentous cyanobacteria such as Merismopedia spp. and Planktothrix spp.; however, due to the very large individual cell size of the C. furcoides, its fresh weight is as high as 86.31 mg/L, accounting for 49.34% of the total algae fresh weight, which is the absolute dominant species in biomass. In addition, by examining multiple specimens collected from northeast China, central China, and southwest China in different periods during high-temperature seasons, based on its own and distribution characteristics, we held that C. furcoides preferred the high temperature and the eutrophic waters, and it has the potential ability to expand and form water bloom in different areas of China.