Internal solitary waves (ISWs) are a common marine internal wave phenomenon in the northern South China Sea that cause significant changes in environmental factors and affect phytoplankton communities. This study investigates the short-term response of phytoplankton communities affected by ISWs based on day–night continuous sampling analysis of the sea area following the passage of an internal wave. The results revealed that, due to the IW-mediated transport of nutrients from deeper to shallower layers, the cell abundance of most small- and medium-volume phytoplankton significantly increased after the passage of ISWs. Using a method based on functional traits, we categorized phytoplankton into four functional groups. Moreover, this study revealed the differences in functional group changes in phytoplankton before and after ISWs. The abundance of mixotrophic phytoplankton in the community decreased, whereas autotrophic and heterotrophic phytoplankton increased.