In order to ensure the safe and economic operations of thermal power plants, heater water levels of high pressure (HP) and low pressure (LP) heaters are important process variables to be monitored in industrial alarm systems. This paper proposes multiple factors affecting heater water levels, and the ways performing the root cause analysis for corresponding alarms. Combining with the operating data of a large-scale 300 MW power generation unit in 2015, the number of each cause is summarized for a large amount of alarms of HP and LP water levels. The results show that excessive movements of control valves are the main causes of alarms for the #7 and #8 low pressure heaters, and changes in power generation load are the main causes of alarms in the #6 low pressure heater. Those analysis results may help operators to obtain the root causes of alarms promptly and take suitable corrective actions to remove alarms for the safe and efficient operation of the high and low pressure heaters.