In order to identify the root reason for the vacuum degradation issues occurred in three 600 MW power plants, field inspection as well as calculation analysis on past operation data has been carried out in this paper. It's concluded that the observed abnormal increment of flow resistance in the condenser may be the main reason. After the implementation of associated measures, the vacuum has been significantly improved. Further, the low speed operation of the circulating pump is considered to have effect on the deposition of the sediment and impurity. The similar plants should pay special attention to this matter.