Abstract: According to the problem of low reheat steam temperature in a natural circulation boiler of a 300 MW coal-fired unit under low load conditions, and combined with the actual operation of the boiler, a flue gas recirculation renovation plan is proposed.It conducts tests under different load conditions after the renovation.The test results show that the flue gas recirculation system can not only effectively improve the problem of low reheat steam temperature under low load conditions of the boiler, but also does it have an upward effect on the main steam temperature.Under different load conditions, the reheat steam temperature increases by about 10～30 ℃,and the main steam temperature increases by about 10 ℃.Based on test data, it analyzes and calculates the economic efficiency of boiler operation.It points out existing problems and proposes suggestions, which can provide reference for power plants with similar problems.