A coupled ECOM-si/N₂P₂ZD model with real-time solar radiation and sea surface heat flux was applied to evaluate the impact of thermal discharge from two coastal power plants (Guohua and Wushashan) on the plankton biomass in Xiangshan Bay. The simulated results show that thermal discharge and its mechanical entanglement and residual chlorine significantly decrease the plankton biomass and change the seasonal and spatial distribution of the biomass. In the warming season, spring and early summer, thermal water makes the occurrence of algal bloom occur earlier than normal. In summer, high water temperature exceeds the optimum growth temperature and leads to a decline of the phytoplankton biomass, and a risk reduction of the red tide. In the cool autumn and cold winter, when the water temperature is lower than the optimum growth temperature, thermal water can increase the primary productivity during this period. In addition to seasonal variation of phytoplankton and zooplankton biomass, annual average biomass of plankton is decreased by the damage of thermal water. The increased amount of detritus decrease the loss of total organism in the sea water, but the changes of annual average total organic matter damaged by thermal water is still negative. The thermal discharge of Guohua power plant impacts greater than Wushashan power plant. Both the range of area with temperature increased and plankton biomass variation, the Guohua power plant impacts the sea area in Tie Port greater than in Huangtun Port.