A large cooling system of residential building in Changsha City in China was investigated in the summer of 2003. The relationships among the controlled variables, uncontrolled variables and the chillers’ performance were obtained empirically with the test data. A model of optimal operation for the system was established based on these empirical relationships. A parameter, system coefficient of performance (SCOP), is presented to analyze the effect of energy saving of the cooling system. The results show that the energy saving is likely to reach as high as 10% by applying the optimal model to the cooling system.