The invention discloses an online optimization method for the secondary side water outlet temperature of a central air-conditioning chilled water heat exchanger. The online optimization method comprises the steps of (1) acquiring the temperature data and cooling load data measured in a central air-conditioning chilled water distribution system at the current moment online; (2) setting the search range and the search step size of the secondary side water outlet temperature set value of the heat exchanger and generating all possible alternative temperature set values at the next moment; (3) establishing a prediction model, calculating the operation energy consumption of a primary side water pump of the heat exchanger and the operation energy consumption of a secondary side water pump of theheat exchanger respectively and adding the operation energy consumption of the primary side water pump of the heat exchanger and the operation energy consumption of the secondary side water pump of the heat exchanger together to obtain the total energy consumption of the water pumps; and (4) determining the optimal secondary side temperature set value of the heat exchanger at the next moment, andminimizing the total energy consumption of the primary side water pump and the secondary side water pump of the heat exchanger. By establishing the prediction model, online optimizing the set value of the secondary side chilled water outlet temperature of the heat exchanger under the current cooling load working condition and minimizing the operation energy consumption of the chilled water pumpson the two sides of the heat exchanger, energy-saving operation of a central air-conditioning system is realized.