A field study has been carried out for one district heating system in Beijing. According to the surveyed data, operation pattern of the district heating system has been discussed and quantitative relationship between supply water temperatures with outdoor temperatures has been developed. It indicates a fine linear correlation of daily temperature of supply water with weighted outdoor temperature within three days and an operation condition of “large flow rate and small temperature difference”. Additionally, the effects of the operation pattern on energy consumption and indoor air temperature have been presented. Results show that the heat supply of this heating system couldn’t meet the exact requirements for different rooms and at different time, and failed to build a stable indoor thermal environment in terms of space and time.