Abstract: Crack appeared on the surface of bond area of the fillet weld joint consisting of spindle and lifting lug after assembly of lower arm of pantograph. Combination with the welded structure and actual manufacturing process, the causes and mechanisms of the crack were analyzed by physical and chemical properties testing of raw materials, T-joint cracking sensibility analysis, macro and micro inspection on the cross section of welded joints crack. The results showed that the crack near the bond line was caused by excessive welding heat input, however, impact force generated through lifting lug assembly that enlarged and expanded liquation crack. Put forwarded solution of crack, to ensure the welding qualification of production.