Abstract: In order to explore the effect of strain aging on welded joints of high strain pipeline steel, the microstructure and mechanical properties of welded joints of high strain pipeline steel before/after strain aging were studied. The mechanical properties of welded joints before/after strain aging were investigated by hardness test, fracture toughness test and digital image correlation (DIC) strain test. The microstructure and substructure characteristics of welded joints were analyzed based on optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD) technology and X-ray diffraction technology. The results indicated that the microstructure of welded joint does not change obviously after strain aging, but the inside micro-defects increase. Meanwhile, the fracture toughness of welded joints decreases, and the decrease amplitude increases with the pre-strain degree. It was found that the increase of dislocation density, strain concentration as well as the geometrically necessary dislocation (GND) density are key factors leading to the decrease of plastic deformability and the deterioration of fracture toughness of welded joints.