Abstract: Energy saving and consumption reduction is the important task of coal-fired power plants, and how to reduce auxiliary power ratio is becoming a key step to successfully carry out the task. When the variable speed condensate pump fail to work at the best situation in lower load, it would increased power consumption ratio. In order to find out the best operating point of variable speed condensate pump, a mathematical model of online analysis for variable speed condensate pump was established based on the law of similitude and the practical performance curve of condensate pump. Considering the pressure loss of pipe, an equations of minimum condensate pump power output was established under the different flows, and the targeted value for pump power was determined in different flow conditions. The minimum power consumption of variable speed condensate pump was calculated when the deaerator operated in constant pressure or sliding pressure, and the optimal operation mode of condensate pump was obtained. The example results show the validity of this method.