At present, most of the design of reinforced concrete bent structures are based on elastic assumption.But in practical engineering, the stress of bent structure is often accompanied by the cracking of concrete, the phenomenon of bond slip between reinforcement and concrete,etc.The second-order effect of the bent structure often aggravates the structural stiffness degradation.Therefore, it is necessary to calculate the second-order effect of material nonlinearity in the analysis of bent structure.The more practical method in engineering design is to introduce the elastic second-order analysis method considering the stiffness reduction, and its accuracy depends largely on the value of the stiffness reduction factor.This paper tries to find a reasonable stiffness reduction factor of the bent structure through the PMM fiber hinge based on the material model.