The effect of ultrasonic impact on corrosion fatigue properties of P355NL1 steel welded joints was studied, while the surface morphology and the fractured surface were characterized by optical microscope and scanning electron microscope. Meanwhile, the electrochemical corrosion rate of the weld joints before and after ultrasonic impact was comparatively examined by electrochemical workstation. The results show that the fatigue strength of the welded joint in the as welded state decreases by 12.5% in 6% (mass fraction) NaCl corrosion solution in comparison with that of the substrate. In 6%NaCl solution and water, the fatigue strength of the ultrasonic impacted welded joints is increased by 75% and 53%, and the slope of S-N curve is changed by 75.4% and 60.4%, respectively, in comparison with that in the welded state. The maximum depth of the plastic deformation layer is about 350 μm. The fatigue life of the welded joints is significantly improved by the ultrasonic impact, correspondingly, the fatigue fracture location is also transferred from the weld toe to the weld or base metal area, and the number of corrosion pits is significantly reduced. The results clearly show that the ultrasonic impact can refine grain, reduce stress concentration, eliminate harmful residual tensile stress, introduce beneficial residual compressive stress, reduce electrochemical corrosion rate, and improve the corrosion fatigue performance of welded joints of P355NL1 steel.