Wire-by-wire and layer-by-layer printing processes used in fused deposition modeling (FDM) three-dimensional (3D) printed parts result in poor mechanical properties. In this study, 3D printed acrylonitrile butadiene styrene (ABS) samples strengthened by ultrasonic vibrations are studied by a controlled variate method. The effects of ultrasonic strengthening pressure and ultrasonic strengthening time on the tensile mechanical properties of samples are studied. The tensile strength of the samples increases by 11.3%, the Young’s modulus increases by 16.7%, and the surface roughness decreases after ultrasonic strengthening. Ultrasonic strengthening after FDM 3D printing significantly improves the tensile mechanical properties of the sample and broadens the potential applications for FDM 3D printing technology.