A dynamic model of microorganism and assimilable organic carbon(AOC) is proposed in the drinking water. According to the realtime experiment data of microorganism, AOC, free chlorine, parameters in the model are checked and the reliability of the model is verified. The experiment results show that the concentration of AOC increases rapidly as time goes by at earlier stage for the oxidation of free chlorine, while in the later stage, AOC is slowly consumed in the progress of microorganism growth as organic food. By using the model, it can be proved that there is optimum initial free chlorine dosage so that the concentration of microorganism meets the requirement of standards at the ending time. It is useful for drinking water plants to save running cost.