The model of CFST arch bridge was established by ABAQUS, and the dynamic characteristics of the arch bridge were analyzed by using the elastic modulus as variables which was calculated by FEM under different defect conditions based on the unified theory. The results show that as the gap rate increases, the frequencies of the arch bridge decrease, according to the modal analysis of the CFST arch bridge under non-gap、spherical gap and circumferential gap as well as under the same gap rate, the frequency of spherical gap are larger than those of circumferential gap. It is concluded that for the same kind of gap defects, the greater the gap rate, the greater the impact on the dynamic characteristics of the bridge and in the problem of gap between steel and core concrete, circumferential gap has a greater impact on the dynamic characteristics of the bridge.