Abstract: 10Ni5CrMoV steel with thickness of 25 mm was welded by MAG (melting pole active gas shielded welding). The microstructure and impact fracture morphology of joint were observed, and hardness, impact property and tensile property were studied. The results show that the microstructure of joint weld zone was mainly lath martensite/bainite+granular bainite, and there was much M-A component distributed along austenite grain boundary. The hardness of the weld zone closed to that of base metal, and average impact energy at -50℃ was 42 J which represented low impact toughness. With the increase of distance from center of weld, the microstructure of heat affected zone was quench martensite, lath martensite/bainite+ferrite+granular bainite in sequence, and the carbide of critical zone accumulated and grew up. Hardness increased first and then decreased with the minimum hardness of 286.4 HV. The impact toughness was relatively high and the average impact toughness was 188 J at -50℃. The tensile strength of welded joint was 920 MPa and the location of fracture in tensile test was base metal. M-A component and carbide were the main factors that affected the performance of joint.