In order to obtain the fatigue stiffness degradation law of in-service concrete T-beams and carry out fatigue life prediction. Based on the theory of damage mechanics， a stepped stiffness model of concrete T-beams including cracking damage was constructed. Through the full-scale model of three 10 m concrete T beams， static and fatigue failure tests were carried out to obtain the evolution law of fatigue residual stiffness with the number of loads. The fatigue damage coefficient and stiffness degradation coefficient were introduced to establish a concrete T beam in service fatigue life prediction model. The analysis results show that with the increase of load， the residual stiffness of the bridge exhibits three-stage decay， in which the initial and later stages of fatigue decay rapidly， but account for a relatively small proportion of the whole life. The result shows that more than 80% of the whole service life is the main stage of bridge service， it is about 82.7% of the initial stiffness at the time of fatigue failure. Finally， according to the working behavior of in-service concrete T-girder bridges， a life prediction method for in-service reinforced concrete girder bridges is proposed， the related research results can provide theoretical support for the study of life prediction of such bridges.