The optimization of cooling water system relates to the energy consumption of water chiller and the operating condition of the chilled water system,so the optimization is in difficulty. The paper adopted the large scale system optimization method to achieve the decomposition and coordination optimization of variable air volume air conditioning systems,and provided the large scale system models of cooling water system,water chiller and chilled water system. Fully considering the influencing factors about the chiller energy consumption, we achieved the energy saving optimization of cooling water system. The energy consumption model of cooling water system and water chiller was established. The results of energy consumption simulation show that the large scale system optimization method can achieve the energy saving operation of cooling water system based on the global optimization. It provides an effective method to improve the operating efficiency of variable air volume air conditioning systems.