Abstract: In order to study the actual stress mechanical properties of RC beam strengthened with TRC sheets，one RC beam and five reinforcing beams whose parameters were the initial stress were chosen as the research objects．Magnesium potassium phosphate materials with high temperature resistance and good interfacial bonding propertiesinorganic cementitious (MPC) were used to study the flexural strength of normal section of RC beam strengthened with TRC sheets based on MPC interface agent and to analyse the impact on the failure characteristics，development of cracks and the mechanical properties of the strengthened beams． The results showed that the precast TRC sheets and RC beams have a better interface bonding performance; strain hysteresis of TRC sheets beam became obvious with beam's initial stress increased，and the ultimate load and stiffness of RC beam decreased; prefabricated TRC thin sheets could improve the form of crack which presented fine and dense cracks on the RC beams． For the smaller injury RC beams，the reinforcement effect was better; for the RC beams nearing the yield of steels，the increase intheir capacity was also limited even reinforced RC beam was unloaded．