Being the most critical component among long-span PC bridges, prestress steel tendon’s current condition is closely related to structural safety and durability. For the sake of measuring it exactly to estimate the condition for prestressing reservation practically in service stage, 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 demonstrate accuracy and practical utility of the technology.