In this paper, a method to estimate the target value of terminal temperature difference of horizontal three-segment feed water heater under various working conditions is proposed. The temperature of the water at the outlet of the condensation segment is obtained through mass and energy balance on the basis of actual extracted steam flow rate. Then by means of energy balance equation and heat transfer efficiency equation, the temperature of the outlet water at maximum heat transfer efficiency is calculated, thus obtaining the terminal temperature difference target value. Results based on offline operation data of a real 1000 MW supercritical power plant are given and the difference between target value and actual operation value could be used to estimate the loss differential of the terminal temperature difference.