The work aims to study the relationship between microstructure and tribocorrosion resistance of fine grain Ti-6Al-4V alloy by friction stir in synthetic seawater. Ti-6Al-4V alloy with equiaxed grains and fine lamellar sub-structure α phase was obtained by controlling the stir friction processes (200 r/min-25 mm/min and 200 r/min-50 mm/min). Tribocorrosion tests were conducted to Ti-6Al-4V alloy with reciprocating tribometer and electrochemical workstation in the synthetic seawater to obtain the coefficient of friction-time curves, potentiodynamic polarization curves, etc. Laser confocal microscope and field emission scanning electron microscope were used to observe the wear scar, calculate the wear rate and analyze the wear mecha-nism. The factors mainly including the material wear was investigated by calculating the corrosion components. During corrosion, the oxide layers of samples were destroyed and the Ti-6Al-4V alloy with fine grains occupied larger boundaries, the corrosion potentials decreased and corrosion current densities were smaller than that of original samples. Coefficient of friction curves of FSPed samples were stable. The FSPed sample with fine equiaxed grains showed the lowest COF and the wear rate after tribocorrosion test decreased by 20% compared with that of original samples. The FSPed Ti-6Al-4V alloy with fine lamellar sub-structure had chaotic furrows due to the influence of anisotropy of micromechanical properties on the travel route of grinding balls. The wear mechanisms of original samples were abrasive wear and corrosion wear, while those of FSPed samples in synthetic seawater were abrasive wear, delamination and corrosion wear. The FSPed Ti-6Al-4V alloy with fine equiaxed grains shows lower wear rate and low friction coefficient during tribocorrosion tests, so the structure characteristics have the best corrosion and wear resistance.