Prestressed anchor cables have been widely used in major hydropower projects in China. Aiming at the problem of service life of these anchor cables used as permanent support, an anchor cable that has been in service for 20 years on the left bank slope of Manwan Hydropower Station is excavated on site. Based on the results of excavation test, the anchor cable was evaluated from the aspects of corrosion characteristics of anchor head, shrinkage of steel strand, anti-rust effect of cement mortar, characteristics of inner anchoring section, changes of chemical and mechanical properties of steel strand and consolidation of cement grouting. The conclusion of evaluation is that the work of anchor cable is effective and the anchor cable should be safe.