In order to study the level variation of No.6 HP (high pressure) Heater in Unit 8 during low load in the Shanghai WaiGaoQiao No.3 Power Plant,the Pandas module of Python was introduced to group and visualize the 10-year operation data from supervisory information system (SIS) in plant level. The variation law of water level of No.6 HP Heater was analyzed based on the variation trends of parameters such as water level,inlet temperature, outlet temperature, and drainage temperature. The results showed that the drainage temperature and inlet temperature were the main factors affecting water level fluctuation. The increased inlet water temperature was the main reason of water level fluctuation under low load. In order to effectively solve the problem of water level fluctuation under low load conditions, it was suggested to reduce the introduction of heat at the inlet of feed pump during operation to reduce the inlet temperature.