Abstract: In this study, we aimed to characterize the photosynthetic activities of Peridiniopsis niei. We applied chlorophyll fluorescence analysis to evaluate the photosynthetic activities at different temperatures, light intensities and pH values. The results showed that both the actual photosynthetic efficiency of photosystem II (Y (II)) and the maximum quantum yield (Fv/Fm) were significantly raised (P0.05) along with the increase in the temperature (7.5-20). The electron transport rate was not interdicted at low temperatures. High photosynthetic activities of the cells were maintained at the temperature 7.5 to 20. At 10 the photosynthetic activities first increased and then decreased along with the increase in the pH value, and the activities reached the maximum at pH 7.3. The order of activities was Alkalescency Neutral Acidity. The rapid chlorophyll fluorescence kinetics curve showed that the reactive center of PSII and the electron acceptor bank were damaged at different pH values except for pH 7.3. These suggested that Peridiniopsis niei might adapt to only a small range of pH values, which could be pH7.0-8.0. The half saturated light intensity (Ek) was 385.5mol photons/(m2s), indicating that Peridiniopsis niei had high tolerance to strong light. Our study suggested that 1) Peridiniopsis niei may tolerate low temperature and high light intensity 800 mol photons/(m2s), 2) alkalescency could be suitable for the photosynthetic activities, 3) acidity may reduce the photochemical efficiency through the damage of photosystem I and II, 4) the high rate of cyclic electron transport may underlie the high photochemical efficiency and counter the damage caused by the high light. The high tolerance to low temperature and intense light enabled Peridiniopsis niei to be the dominant species, which could be the simplest explanation of Peridiniopsis bloom.