Abstract: Cracks in welding seam were detected in TA2 titanium welded pipe for a chemical project after flattening test. The main reason of cracking was studied by metallographic structure, chemical composition, mechanical properties,fracture analyses of the samples taken from the crack site and simulation calculation of flattening test. The results show that uneven deformation at the weld site in the flattening test process occurs due to high strength and stiffness of the weld,and the deformation is highly concentrated at the weld root. Due to the coarse grains of the weld and heat-affected zone,the plasticity is low. When local deformation at the weld root exceeds the plastic limit, the cracking occurs. The degree of deformation concentration at the weld root is related to the ratio of height to width of the weld. It can be deduced that reducing the weld reinforcement can decrease the strain concentration at the weld root during the flattening test and thus can avoid the cracking.