In the reliability assessment and life prediction of existing reinforced concrete bridge in service, the resistance is an important factor for reliability calculation. So it is very important to establish resistance model. Aimed at the problem of building the resistance decay model of reinforced concrete bridge, the paper analyzed three factors such as concrete strength change,steel bar corrosion and the change of bond capability between concrete and steel bar and their time varying regularity . Considering the decay of steel bar mechanics property caused by corrosion was considered, it introduced the computational model of uncertainty parameter based on fiber model. The resistance decay model of reinforced concrete bridge was built by mathematical method.The paper introduced a simple bridge of existing reinforced concrete to calculate the resistance model, and verified the reliability of the model. The building of resistance decay model can lay the foundation for the reliability assessment of bridge.