The invention provides an on-line estimation method for target values of an all-condition upper end difference and lower end difference of a heater. The method comprises: step one, real-time data of a related measuring point at a given time are obtained; step two, calculation is carried out to obtain on-line estimation values of steam extraction quality flows of all heaters; step three, a heat transfer coefficient from a steam side to a heat exchange tube wall outside a heat exchange tube and a heat transfer coefficient of an aqueous phase working medium to the heat exchange tube wall inside the heat exchange tube are calculated, so that a theoretical maximum value of a total heat transfer coefficient of the heater is obtained when the dirtiness resistance of the heat exchange tube wall surface is zero; step four, a theoretical value of a dewatering outlet temperature of the heater is calculated, definition is carried out by a lower end difference and then a target value of the lower end difference of the heater is obtained; and step five, a theoretical value of an aqueous phase working medium outlet temperature of the heater is calculated, definition is carried out by an upper end difference, and thus a target value of the upper end difference of the heater is obtained. According to the invention, influences on the heat economy of the set by the upper end difference and the lower end difference of the heater can be analyzed; and a support condition is provided for real-time evaluation of heat economy of the heat regeneration steam-extraction system.