The diversity and spatial-temporal distribution of phytoplankton HAB species, contamination status of oyster toxin, and their sources were investigated in Jiangmen oyster farming area based on morphological observation and liquid chromatography-tandem mass spectrometry analysis. The results revealed there were 28 HAB species, including 19 harmless HAB species, two toxic species (Akashiwo sanguinea and Karenia brevis), and seven toxin-producing microalgae (Alexandrium pacificum, Dinophysis caudata, D. miles, D. fortii, Gonyaulax spinifera, Gymnodinium catenatum, and K. mikimotoi). The mean abundance of total HAB species generally showed a trend of increasing from winter to autumn. The total average abundances of toxic HAB species were 269, 265, 321 and 2.6 × 10