Abstract: 10CrNi3MoV steel is welded by metal active gas welding (MAG) with different heat input. The effect of heat input on microstructure and mechanical property of the welded joints are investigated by means of microstructure analysis, fracture observation and mechanical tests. Results show that the microstructure of the welded metal is mainly acicular ferrite, with a certain amount of proeutectoid ferrite and granular bainite, when the heat input (E=11.0 kJ·cm^-1, and E=14.4 kJ·cm^-1) is low. When the heat input (E=18.1 kJ·cm^-1) is high, the amount of acicular ferrite decreases, and the amount of proeutectoid ferrite and granular bainite significantly increase, and the microstructure coarsens. With the increase of heat input, the grain size of the coarse grained heat affected zone (CGHAZ) increases, and the microstructure transforms from lath martensite to lath bainite. The lath boundaries become blurred and the granular bainte emerges. The strength of the welded metal decreases, and the impact toughness increases slightly at first and then decreases significantly. The fracture mode changes from microporous polycondensation ductile fracture to quasi cleavage/ductile mixed fracture. When the heat input is 14.4 kJ·cm^-1, the microstructure is mainly fine acicular ferrite and the welded joint has the best combination of strength and toughness.