The effect of hydrogen addition on compression deformation behaviour of Ti-0.3Mo-0.8Ni alloy argon-arc welded joint has been investigated.Evolution mechanism of hydrogen-induced flow stress was discussed in detail.The results show that with increasing hydrogen content,the stretching and bending extent of fully lamellar microstructures including α lamellas and acicular hydride continued to increase,the morphology of dynamic recrystallization (DRX) grains tended to change from approximately equiaxed to large lamellar shape,and the quantity of DRX grains and recrystallization degree of grains increased obviously.A large number of dislocations concentrated in the vicinity of the hydride.Steady stress was decreased continuously with increasing hydrogen content,while peak stress of the hydrogenated 0.12 wt.％ H weld zone was decreased to the minimum value and then increased slowly.A slight decrease in flow stress of the hydrogenated 0.05 wt.％ H weld zone was caused by limited increase in the volume fraction of softer β phase.Hydrogen-induced DRX of α phase and improved dislocation movement by strong interaction between the hydride and dislocation directly resulted in a sharp drop in flow stress of the hydrogenated 0.12 and 0.21 wt.％ H weld zone.Solute hydrogen also finitely contributed to a sharp drop in flow stress of the hydrogenated 0.12 and 0.21 wt.％ H weld zone by promoted local softening,which induced continuous DRX and more movable dislocations to participate in slipping or climbing.The reinforcement effect and plastic deformation of the hydride and solution strengthening of β phase induced by solute hydrogen finally led to the increase in flow stress of the hydrogenated 0.21 wt.％ H weld zone in its true strain range from 0 to 0.36.