Nonlinear seismic response analysis was carried out to study the dynamic response and damage mechanism of long span steel through arch bridges which were subjected to strong earthquake. Taking the Beijing-Hangzhou canal bridge in Xuzhou as an example, the seismic response and damage of the bridge under strong earthquake were calculated. The results showed that steel through arch bridges had preferable seismic resistant ability and yielded only limited damage under strong earthquake action. The moment for out of plane significantly increased when considering the geometric and material nonlinear state, while the effect of the geometric and material nonlinear state on the transversal and the vertical displacement response was consistent. Steel through arch bridge was prone to be damaged at arch base, lateral bracing and arch rib. Lateral bracings were the weakness of the structure under the lateral earthquake action. We should pay more attention to these weak parts in case this type of bridge is built in a high earthquake intensity area.