In order to make much detail and more practical evaluation on structural safety and durability for long-span PC bridges, the condition for prestressing reservation in service stage turns out to be a necessary key index. For the sake of measuring it exactly, an invented instrument named Stretching Force Tester is applied to monitor the effective prestressing force of strand. The precision of this technology is guaranteed by means of amending analogous boundary condition and minimizing test errors through the tool of double-stage differences. Based on comparatively sophisticated prestressing loss method in current code, an evaluation model on attenuation of effective prestress is built up. By means of obtained stress datum on strand, recognition on two types of nominal coefficients for prestress losses is realized and regularities of practical distribution can be simulated simultaneously. The experimental results of a large-scale beam model show that the relative error for cable tension force between test value and standard one is able to meet the needs of engineering use and general principle for effective prestress can be demonstrated. Therefore, it laid the foundation for inspection and evaluation on structural deterioration of existing long-span PC bridges.