Buffer water tank is an important part of the closed chilled water system of commercial air conditioning units, which can ensure the low water volume required by the normal operation of the system, especially in the application environment where some industrial processes require high temperature change. Usually, the whole air conditioning water system is independent of the air conditioning host, but the unit with its own hydraulic module component is a new development trend, it integrates the pump, expansion tank, buffer tank, water pipe, valve and so on, so that customers no longer spend more energy for the design, layout, installation and other problems of the water system. In practical engineering applications, people often configure buffer tank size based on engineering experience. It takes 3~6 gallons of water per ton. It takes 6~10 gallons of water per ton if the temperature control accuracy is high. Then the volume of water required by the whole system can be calculated, and then the volume of the buffer tank can be obtained by removing the volume of other water storage components in the system [1]. This algorithm is not accurate enough to determine the buffer tank volume of hydraulic module. Because the size of the unit is required, the size of the buffer tank is also constrained.