Abstract: In order to further explore the economic operation modes of circulating water pump in direct seawater cooling units of large coastal thermal power plants , the mathematical model of the circulating water system under the influences of tides is established based on experimental studies and online optimization calculation method. Also by virtual of the data pool method, the circulation water pump operation mode is optimized step by step. The optimization objective is set to be the minimization of online coal consumption. In this way the model for the pump operation optimization is established, such that the optimal operation modes under different tide, seawater temperature and load conditions are obtained. Meanwhile, based on unit coal saving forecasts under different pump operation modes considering the loading conditions of the generation units, the optimal pump operation modes in future time intervals are obtained to provide guidance for the practical operation. The application of the online optimization method in Chaozhou Power Generation Company shows that the circulating water system has achieved very good energy-saving effects.