Hydrogenation of Ti-6Al-4V titanium alloys was carried out in a special furnace through the accumulated flux method, and OM, XRD and TEM techniques were used to investigate the microstructure evolution of the hydrogenated Ti-6Al-4V titanium alloy. The hydrogenation of the Ti-6Al-4V titanium alloy with the hydrogen content of 0.30 wt% makes the α phase fraction decrease compared with that of the as-received Ti-6Al-4V titanium alloy, but it makes the β phase fraction increase. The δ hydride (TiH2 phase) occurs in the hydrogenated Ti-6Al-4V titanium alloy when the hydrogen content is more than 0.3 wt%. The shear deformation is main pattern in the transformation processes from β-Ti(H) phase to α-Ti phase and δ hydride (TiH2 phase). The phase transformation temperature of the hydrogenated Ti-6Al-4V titanium alloy decreases by 180 °C compared with that of the as-received Ti-6Al-4V titanium alloy, and it is related to the phase fraction and phase transformation.