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A typical district cooling system (DCS) with a chilled water storage system is analyzed in hot summer and cold winter area in China. An analysis method concerning operation modes is proposed based on measured data, which is obtained by long term monitoring and on-site measurements of cooling season. The DCS operates at partial load for a large proportion of the cooling time; in particular, the partial-load rate (PLR) can be less than 25％ for more than 50％ of the total cooling season. In the night, PLR reaches 5％ of the peak load. Thus, it is critical to achieve efficient operation under partial-load conditions of the DCS. Installation of chilled water thermal storage presents a solution to improve the working condition of the DCS and chillers. From the beginning to the end of the cooling season, the DCS operation can be summarized by typical operation modes according to cooling demand and chiller operation. For each mode, the base-load chiller operated at a high-load rate, with an average value of 0.88, and the coefficient of performance (COP) remained in a small range, between 4.2 and 5.2. The average energy efficiency ratio (EER) of the DCS for the cooling season was 3.65 and 3.81 for Years A and B, respectively. With respect to the economics, chillers used 90.2％ of off-peak electricity, at only half the price of peak electricity.