Abstract: The microstructure of 3Cr low alloy pipeline steel welded joint was analyzed by scanning electronmicroscope (SEM). CO₂ corrosion behavior of 3Cr low alloy pipeline steel welded joint was studied with high temperature and high pressure autoclave and electrochemical impedance test technology. The weld metal and the base matal of 3Cr low alloy pipeline steel have the same chemical compositions, and consist of ferrite and pearlite.The microstructure of the weld metal is finer and better-distributed. Weight loss test results show that the corrosion rate of weldrf joint less than that of base metal by 0.28 mm/year. Electrochemical impedance test results show that the resistance of the weld surface corrosion film is greater than that base metal after soaking for 48 h and 168 h, and the protection to the base metal by weld corrosion product is better than that by the base metal, which caused by the weld microstructure are even and small, the surface corrosion film is compact and uniform than that of the base metal surface.