Abstract: The spiking is the unique defect of electron beam welding, which has a serious impact on the welding quality.The academia still hadn't had an unified understanding of the forming mechanism of spiking.To study the forming mechanism of spiking, the orthogonal test was used to carry out the experiment of titanium alloy electron-beam deep-penetration welding.The X-ray detection was done for each weld after welding The results of X-ray detection show that the spiking only exists in the partial penetration weld, and the spiking is the irregular-slited shape while the roots are round.The optical microscope and scanning electron microscope combined with energy dispersion spectroscopy were used to analyze the formation mechanism of spiking.The results show that the pulse of electron beam is the direct cause of spiking formation and the metal vapor with high saturated vapor pressure accelerates the tendency of forming spiking.It is important intrinsic motivation to the generation of spiking.