Extensive multi-species harmful algal blooms (HABs) were triggered by Super Typhoon Lekima in Laizhou Bay (Bohai Sea) in August 2019. After conducting two field cruises before and after the typhoon passage, we employed both high-performance liquid chromatography (HPLC)-pigment and microscopic methods to study the changes in the phytoplankton community and biomass. Following the passage of Lekima, the average surface salinity decreased, while dissolved inorganic nitrogen and dissolved silicate concentrations increased in the study area. The phytoplankton abundance and Chl a significantly increased after the typhoon event. Post-typhoon, the highest abundance values of Pseudo-nitzschia spp., Noctiluca scintillans, and Coscinodiscus spp. reached 10⁶ cells/L and those of Bacillaria paxillifera, Ceratium spp., and Gymnodinium catenatum were in the order of 10⁵ cells/L. HPLC-pigment CHEMTAX analysis showed that the biomass (Chl a) of dinoflagellates, diatoms, cryptophytes, chlorophytes, and haptophytes increased significantly after the typhoon. The increase in Chl a concentration was mainly attributable to large-sized phytoplankton, which are mostly diatoms and dinoflagellates. This study highlights that typhoons may cause HABs by introducing large amounts of freshwater and nutrients and change the phytoplankton community in a temperate and inner bay.