Abstract: In order to improve the life of the unit, and study the water erosion of the last stage moving blade caused by the deep peak shaving of steam turbine, this paper established a method to measure the water erosion of blade based on relative volumetric flow. The calculation analysis with operating experiences of the unit was employed. The influence of back pressure and load on water erosion comprehensively was taken into account. The results show that, with the decrease of relative volumetric flow rate, the water erosion part of the rotor blade is transferred from the top steam inlet side with lower stress to the root steam outlet side with higher stress. It results in the increase of risk. With the decrease of relative volume flow, the water erosion rate of moving blade shows three stages: first increasing, then decreasing, and rising finally. The turning point between stages corresponds to a balanced state.