In view of measuring strain on the structure surface by the bare fiber Bragg grating（FBG）, and based on the strain transfer model of fiber layer-cement layer-matrix layer, the prestressing effect on the strain transfer rate is studied theoretically and experimentally. The strain on both positive and negative sides of the equal strength beam is measured by means of the prestressing FBG as well as the normal FBG. In order to eliminate the effect of the length and thickness of the cement, the LOCTITE dispenser is used to control the amount of cement. The curve of strain changing with load is obtained. And through comparing with the theoretical strain value of equal strength beam, the curve of strain transfer rate changing with load is received. The results show that the rate of the prestressing FBG is higher than that of the normal FBG for tensile strain mearsuremnet, both of which are above 96%, while the rate of the prestressing FBG is lower than that of the normal FBG for compressive strain mearsuremnet, both of which are above 95%. It corresponds with the theoretical analysis. It proves that the strain transfer rate is improved thanks to the prestressing in the experiment of tensile strain. On the contrary, the rate is reduced because of the prestressing in the experiment of compressive strain.