In this paper, 3D-printed Ti-6Al-4V (Ti64) alloy, a hard-to-deform alloy with poor surface finish and high porosity was processed by an innovative technique, laser-assisted ultrasonic nanocrystal surface modification (LA-UNSM). The localized thermal effect of continuous laser decreases the flow stress of the alloy effectively. The effects of LA-UNSM on surface finish, microstructure, and surface hardness were investigated and compared with traditional UNSM treated samples. It is found that much smoother and harder surface was obtained after LA-UNSM. In addition, LA-UNSM also led to a thicker plastically-affected layer compared with UNSM process only. It is found that the LA-UNSM is a very efficient post-processing technique for 3D-printed alloys since it makes the thermal and mechanical effects work synergistically and energy-efficiently.