Research purposes ： In order to overcome the stress diference between the new and old materials caused by the diferent stages of the bridge reinforcement，a new type of structural prestressed reinforcement method was proposed. Based on the limit state design，optimum unload rate was defined，then on this basis，the bearing capacity formula was deduced. Also three beams with same size was reinforced under different unload rate，and the bearing capacity of combinational section was measured，the test data validated the derived results. Research conclusions:(1) This method takes into account the characteristics of the structure with reinforcement，which can effectively change the stress difference between the old and the new material. (2) The unloading rate has a significant effect on the load bearing capacity of the normal use limit state，but little on the ultimate bearing capacity ： When the unloading rate is 46%，the normal use limit state of the test beam and the bearing capacity limit state carrying capacity are increased by 45% and 4% respectively; When the unloading rate is 213% ( reverse unloading)， the normal use limit state carrying capacity is reduced by about 14%，ultimate bearing capacity increased by about 10%. (3 ) The research results can be used to optimize the unloading rate and parameters of reinforcement design.