There is much exergy loss in low-pressure feed water heater system of the thermal unit. The thermo-economic characters by the equivalent enthalpy drop method while the low-pressure heater was replaced with the ejector heater. At the same time, the thermo-economic characters formulas for the different factors were also gotten. Based on these results, the thermo-economic changing features were analyzed for the low pressure heaters replaced with the ejector heaters of the N600-165/535/535 units, and the results were compared with the normal heat balance method. The results show that the ejector heaters can reduce the exergy loss efficiently, and the thermo-economic effects is the best for replacing the all low-pressure heaters with the ejector heaters, and its efficiency can be increased by 0.263%. The given equations of this paper can be used for analyzing the thermal economic effects.