Hydrogenation and nanocrystallization of Ti6Al4V alloy were achieved by electrolytic hydrogenation and SMAT processing. The hydride δ phase appeared when the hydrogenation time reached 4 h. The grain size of Ti6Al4V alloy after surface nanocrystallization reached below 35 nm when the SMAT processing time was 2 h. Surface morphology and roughness were characterized by AFM. The diffusion bonding between Ti6Al4V alloys at low temperature was achieved under different parameters. It is demonstrated that the porosity at the interface decreased correspondingly when the hydrogenation time or nanocrystallization time was extended. The porosity of the joint reached 0 when the hydrogenation time was 24 h or the nanocrystallization time was 4 h. Then hydrogenation and nanocrystallization treatments were combined. It is found that there are more bonding defects in both hydrogenation-nanocrystallization and nanocrystallization-hydrogenation joints than in joints with single hydrogenation or nanocrystallization treatment. It can be concluded that the porosity of the joints with nanocrystallization first is lower than that with hydrogenation first.