Under urbanization trend in China，the public transportation grows fast in recent years. As one of the comfortable and quick way, Subway is being constructed in more and more cities. Now, most of the subway station is equipped with HVAC systems. The increasing energy consumption of HVAC systems takes a large part, which becomes a serious problem. In this article, the author has conduct field measurements in typical subway station HVAC systems in Wuxi, a city of Jiangsu province in China, then finds out the excess of fresh air is to blame. The excess of fresh air leads to the increasing cooling load of the subway station HVAC systems. To make the subway station HVAC systems more effective, the author analyses the possible solutions. The results may provide guidance to both design and operation for subway HVAC systems.