The samples of ballast water were taken at Shanghai Yangshan Port from April 2015 to July 2016 from nine vessels which had sailed along 21st-Century Maritime Silk Road. The seawater was ballasted in South China Sea area, Arabian Sea area, Red Sea area and Mediterranean area. The biodiversity and abundance of phytoplankton were analyzed. In this survey, 30 species of phytoplankton were detected in ballast water, belonging to 4 phyla and 18 genera. Among them, there are 21 species under 12 genera of Bacillariophyta, 7 species from 4 genera of Pyrrophyta, and 1 species of Cyanophyta and Chlorophyta, respectively. The number of species of phytoplankton in ballast water varies from 2 to 7. The dominant species were Hyalodiscus stelliger, Melosira sulcata, Skeletonema costatum, and Dinophysis caudata, among which Dinophysis caudata has the highest frequency. The highest abundance of phytoplankton was (3.983±1.695)×10³ cells/L, sampled from the seawater ballasted in Hongkong in spring. The lowest abundance of phytoplankton, (0.317±0.347)×10³ cells/L, was from the samples of Singapore water ballasted in summer.In addition, 13 species of red-tide algae were identified, including 3 species of toxic algae from 8 ballast water samples. The results showed that the biodiversity and abundance of phytoplankton in the ballast water samples from the South China Sea area were higher than those in other sea areas. Ballast water in all four sea areas has the potential for biological invasion. If some red-tide algae species are released into the Yangshan Port area upon discharging ballast water, the potential risk of red tides will increase, which will endanger the ecological environment of local sea areas.