The surfaces of X70 pipeline steel welded joints are strengthened with laser shock wave. The test of impact toughness at low temperature is carried out. The available grain sizes of the structures by laser shock processing are analyzed, and its fracture morphologies are observed with scanning electron microscope (SEM). The affecting mechanism of laser shock processing on impact toughness of X70 pipeline steel welded joints is investigated. The experimental results show that uniform refined ferrite grains are obtained in the surface of X70 pipeline steel welded joints by laser shock processing, which has excellent impact toughness. The fractures of X70 pipeline steel welded joints in primitive state exhibit embrittlement characteristics, while those after laser shock processing exhibit strengthening characteristics. The impact properties of X70 pipeline steel welded joints at low temperature are improved by laser shock processing, which is beneficial to increasing the usage security of pipeline steel transmission.