A gradient nanocrystalline structure of high hardness with a thick severe plastic deformation (SPD) layer was prepared on the surface of the titanium alloy by changing the pressure or time of shot peening. The deformation layer thickness and surface grain size of the gradient nanocrystalline TC4 titanium alloy were adjusted by changing the shot peening pressure from 0.3 MPa to 0.6 MPa or the shot peening time from 15 min to 60 min. Metallographic microscopy was performed to observe the cross-sectional microstructure of plastic deformation layer. The grain size of the surface nanocrystals was determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hardness of plastic deformation layer was studied by microhardness tester. The experiment results show that under certain shot peening pressure (0.6 MPa), the thickness of the SPD layer and the total deformation layer reached their saturation value of 78 μm and 143 μm respectively after 25 min and 30 min. Comparing the results shot peened under different pressure for 25 min, the thickness of both the SPD layer and the total deformation layer increased with rising pressure and reached their maximum value of 78 μm and 120 μm at 0.4 MPa, respectively. When the SPD layer was saturated, the surface grain size and hardness enhancement tended to be stable. When the surface α phase was refined into a stable state at 0.6 MPa, the surface grain size ranged from 30 nm to 90 nm, and the surface hardness was increased by about 30%. In summary, the thickness of both the SPD layer and total deformation layer increased in a step wise manner, with the increasing shot peening time or pressure, before they reached the saturation values. It should also be noted that both the surface grain refinement and surface hardness reached a saturated state before the thickness of the SPD layer was saturated.