Abstract: Study of prestress loss is one of the key technologing in prestressed FRP reinforcement． This paper presented a new type of stretching and anchoring integral anchorage for prestressed FRP reinforced damage internal pressure steel pipe，analysed the generation mechanism of friction loss，anchorage loss，relaxation loss caused by the strengthening process． By elastic-plastic mechanical plane stress solution and friction loss test research，the stress distribution model of fiber sheet after the friction loss was established． Based on the analysis of relaxation loss test and anchorage loss test，it was found that the anchorage loss existed in the first 80 minutes of the process and that the anchorage loss was related to the locking degree of bolt． Besides the relaxation loss percentage and the change law of relaxation loss with the time were also got．