Well casings of Puguang Gas Field took place abnormal deformation before and after Wenchuan Earthquake, but the conventional theory was unable to explain it. By seismicity and seismic load analysis, it was concluded that Wenchuan Earthquake was an important factor to the deformation. Seismic load caused static stress change,dynamic load, fault slip, inconsistent deformation between stratums, salt/gypsum creep speedup and casing deformation. The analysis showed that not only the stress size affected the deformation, but also the stress change imposed important influence. Drilling in seismic area should pay attention to the effect of structural stress change.