Take the Wuhan Junshan Yangtze River Bridge as a example, whose main-span is 460m, the cable force of completed bridge were calculated by the self-developed program，and the dynamic character of it was analyzed. The results show that: 1) Its first mode of vibration is longitudinal floating mode, which is favorable to the earthquake- response of structures. 2) Its former 40 rank frequency are located between 0.1~2Hz which is avail to the condition of traffic condition.3) the ratio of torsion and bending-torsion frequency is 2.04 which is very high. It indicates the flutter critical wind velocity is high and the windproof performance of this bridge is good.4) the dynamic responses are sensitive to the connection mode which should be simulated correctly. 5) In this kind of half-floating cable-stayed bridge, the vibration of the tower contributes most to the transverse seismic response of the tower. The conclusion can give a reference to the seismic analysis of this kind of bridge.