The superplastic forming and diffusion bonding (SPF/DB) of hydrogenated Ti-6Al-2.8Sn-4Zr-0.5Mo-0.4Si-0.1Y (Ti600) alloys were carried out in the temperature range of 1073-1213K under 1.5MPa gas pressure. The effects of hydrogen contents and diffusion temperature on welding-on ratio of SPF/DB and microstructure of interface and matrix in Ti600 alloy were investigated by OM and SEM. According to the experimental investigation, when the parameters of SPF/DB were as follows: T=860°C, P=1.5MPa and t=70min, the welding-on ratio of Ti600 alloy with hydrogen 0.5wt% was 100 percent. However, the physical contact of Ti600 alloy without hydrogen which was related to plastic forming could not occur. Moreover, the size and amount of voids at the diffusion bonding interface decreased and diffusion bonding quality improved gradually with the increase of hydrogen content and diffusion temperature, which was attributed to the decrease of phase transformation temperature and flow stress of plastic forming as well as the release of hydrogen. After SPF/DB, the recrystallization of joint grains through the interface was formed, and the matrix of hydrogenated Ti600 alloy changed.