Abstract: Water and steam corrosivity to metal materials is mainly evaluated by monitoring the water quality index, but the water quality index fails to directly reflect the real corrosion state of the metal materials. An electric resistance probe installed in the location where boiler economizer sample tube was located was used to on line monitor the corrosion losses of the feedwater system metal materials during boiler operation and shut down protection of a 600 MW subcritical unit. The influences of the boiler operational conditions on the measurement of the corrosion rates were studied by analyzing the influences of the boiler operational conditions on the measurement of the curves of corrosion losses The reliability of measured data was studied by comparing and analyzing the changes of corrosively index of feed water quality and the changes of measured corrosion rates. The results show that: 1)The measured corrosion loss curves will be shifted up or down when boiler operation conditions were changed. The corrosion rates corresponding to corrosion loss curves under different operation conditions shall be calculated in a stepwise manner,so as to reduce the impact of the boiler condition change on corrosion rate calculations. when we calculate corrosion rates using the slopes of corrosion loss curves i because piecewise calculation of corrosion rates ca; 2) The on line corrosion monitoring system can monitor the corrosion rate of the metal materials in feed water system during the time of boiler operation and shut down protection. The monitoring results provide reference for the application of both feed water treatment methods of operating boilers and boiler shut down protection methods.