Abstract: To accurately detect under-anchorage effective prestress， a test system for prestressed steel strand was developed and its working mechanism was revealed based on the equivalent stiffness theory. A specific criterion of tension for two kinds of situations was given and the tension-elongation characteristic curve was explained. Results show that tension-elongation curve can be divided into inelastic deformation eliminating section， elastic deformation of the reverse stretching section and co-work of both reverse stretching section and anchorage section. Specific tension criterion can be given according to whether there exists an inflection point in the tension-elongation curve， which can be used for detecting under-anchorage effective prestress. The error between actual working tension and the tested tension is less than 3%. It is suggested that the prestress loss should be considered in the evaluation standard of under-anchorage effective prestress.

Key words: under-anchorage effective prestress, construction period, equivalent stiffness, test system, criterion