The surface nanocrystallization of a Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) titanium alloy with a lamellar microstructure was carried out by supersonic fine particle bombardment (SFPB). The effect of SFPB gas pressure on its surface integrity, microstructural evolution and mechanical properties was systematically investigated. The results showed that gradient nanostructures on the surface of the TC11 alloy were successfully created after SFPB with different gas pressures. The grain size of the surface’s lamellar microstructure was completely refined to the nanometer scale. And the grain size of nanocrystals decreased with the increase of gas pressure. Meanwhile, the subsurface retained initial lamellar microstructure morphology. Surface roughness was minimized after SFPB with a gas pressure of 1.0 MPa, while microcrack formed at a higher gas pressure of 1.5 MPa, resulting in a decrease of compressive residual stress. With the increase of SFPB gas pressure, the surface microhardness and the depth of hardened layer gradually increased, and yield strength and tensile strength was improved. Nevertheless, the elongation was not greatly changed. The fracture morphology changed from typical ductile fracture to quasi-cleavage and ductile mixed fracture.