A constant temperature economizer comprises an economizer body, the economizer body is externally connected with a down pipe to form a closed cycle, an external heat exchanger is arranged on a circulating pipeline, when water is adopted as a circulating medium in the down pipe, a vacuum pump is arranged on the external heat exchanger to control operating pressure between 50, 000 Pa and 80, 000 Pa, the wall temperature of the economizer body is maintained between 80 DEG C and 95 DEG C, and ethyl alcohol can be directly adopted as a circulating medium in the down pipe. The wall temperature of the economizer is maintained between 80 DEG C and 95 DEG C constantly, low-temperature corrosion does not occur, the flow rate of working media in a heating surface is low, resistance is small, and no in-line pump is needed; the flow rate of flue gas is designed to be lower than 7 m/s, and the abrasion rate of the heating surface is low; the temperature of the flue gas is reduced, specific resistance of flying ash is lowered, the dust collection area of a bag deduster is enlarged, the dust removal efficiency of electricity and the dust removal efficiency of bags are both improved, flue gas waste heat is used for heating heat supply network water, cold air and condensed water, the waste heat is made full use of, and economical efficiency is remarkably improved.