Abstract: An analysis has been carried out on water erosion features of the last stage blade in an extraction condensing turbine unit, based on which the original empirical formula for calculation of moisture loss is proved to be insufficient. Moisture loss generally comes from following two aspects: the preliminary water droplet and the secondary water droplet. When the steam moisture is higher than a certain value, the loss by secondary water droplet will be greater than that by the preliminary one. Meanwhile, the preliminary water droplet formed by nucleation condensation will also cause water erosion on the blades. The moisture loss may be reduced by using outer-edge inner-surface dewetting boards, with which the unit efficiency will correspondingly be raised and the service life of blades can be elongated.