Abstract: In order to recognize the reason of steel structure cracking, by analyzing steel structure's welding method (SAW,FCAW,SMAW) and some weld seam of different position's testing by MT, it was concluded that the defects hade some relations with welding method, welding position, welding procedure conditions, welding stress, assembly and so on. So the reason to cause defect and its position should be known before establishing WPS.