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An ecological dynamic model for the simulation of two pelagic phytoplankton groups is developed in this article. Model parameters were adjusted and validated based on the light-limited field culture experiments and the mesocosm experiments in the East China Sea (ECS). The calculation comparisons from the proposed model, along with field experiment observations, show that the model simulate the datasets very well, qualitatively and quantitatively. The parameters’ sensitivity analysis indicates that the competition between the diatoms and dinoflagellates is most sensitive to the photosynthetic process, followed by the exudation process of the phytoplankton, while the autolysis and respiration processes of phytoplankton and the grazing and exudation processes of zooplankton can also influence this competition to some extent. The sensitive parameters include: the photosynthetic optimal specific rate; the optimal irradiance and optimal temperature for phytoplankton growth; and the half-saturation constant for limiting nutrients, etc. Results of the sensitivity analysis also indicate that light, temperature and limiting nutrients are the controlling environmental factors for the competition between the diatoms and dinoflagellates in the ECS. In order to explore the effects of light and nutrients on the phytoplankton competition, simulations were carried out with varying light and nutrient conditions. Model simulations suggest that the diatoms favor higher irradiance, lower DIN/PO4–P ratios, higher SiO4–Si/DIN ratios and higher nutrient concentrations, as compared to the dinoflagellates. These results support the speculation that the increase in the DIN/PO4−P ratio and the decrease in the SiO4–Si/DIN ratio in the ECS may be responsible for the composition change in the functional Harmful Algal Bloom (HAB) groups from the diatom to the dinoflagellate communities over the last two decades. [ABSTRACT FROM AUTHOR]