The draw-wire displacement sensors to monitor the longitudinal displacement of the main girder of Jiangyin Bridge were introduced in the upgrading projects of the structural health monitoring system. The draw-wire displacement sensors were placed at the two ends of the main girders. The longitudinal displacements at the end of the main girder are being real-time measured when the draw-wire moves with the main girder. The sensors were installed on the cushion caps at the southern and northern ends of the main girder. One sensor was installed at each eastern and western side of per cushion cap, so the total sensors were four. The signal of the sensors was transmitted through the communication cables. The sampling frequency is 50Hz. The displacement changes between the two ends of main girder and the synchronism of displacement at the eastern and western sides are acquired by analyzing statistically the measured data. On the accident of the impact by ship, the displacement response and the excited status of the bridge were collected to evaluate the damage of the structure. The research results show that the variation value of the daily displacement of the main girder is small but the frequency of the movement is high, the displacement at the eastern and western sides has a good synchronism. The high-frequency vibration of the structure occurred after the impact of ship but the amplitude of vibration was relatively smaller than the daily displacement variation, so the impact has small influence on the structure and the structural rigidness of the main girder has no obvious deterioration after six-year services.