Abstract: Ttransformation super-plastic welding (TSW) is an unique welding method, which takes advantage of material's super-plastic properties of low flow stress and high plasticity. In this paper, TSW model based on elastic visco-plastic mechanics had been established. Numerical simulation results show that the stress state of border will be changed from compressive to tension within the TSW joints with free border with the increasing of expanding ratio. Not only the position of tensile stress, but also its value will be influenced by welding upsetting pressure and thermal cycles. The two parameters' suitable matching is the key factor influencing the quality of the joint. Q235/Q235 TSW process experiments show that suflficient bonding is limited to certain scope of the middle area of joints and defect such as the opening is inclined to emerge at the edge. The results show that the position of the opening depend on the stress of the interface.