The dynamic behaviour of natural vibration characteristics of the Eastern Bridge in Changzhou city was studied. The ANSYS was adopted to set up a three-dimensional finite element model for a flying swallow type CFST arch bridge of the main bridge. The natural periods and modes of the CFT arch bridge were calculated using the subspace iteration method. Meanwhile its modal characteristics were analyzed. The results showed that the stiffness of bridge in- plane was smaller than that of out-plane. The calculating results could offer the related technical parameters and basic data for the design and the construction of the bridge as well as the health detection and maintenance of the bridge in working conditions.