Abstract: The 3Cr weathering steels were welded by MAG with three heat inputs, and the effects of heat input on microstructure and properties of the weld metal were investigated by means of optical microscopy, scanning electron microscopy and transmission electron microscopy. The results indicate that the microstructure of the three weld metal consists predominantly of lath bainite, granular bainite and M-A constituent. With the increasing of the heat input, the amount of granular bainite and M-A constituent increase, and the size of M-A constituent increases. The impact toughness of weld metal drops with the increasing of heat input. Microstructure coarsening and the increasing of the content and size of M-A constituent are the main reasons of toughness deterioration. The welded joints show nearly the same corrosion resistance property for different region, and the corrosion product consists predominantly of α-FeOOH. Welded joints with good properties can be obtained with the heat input between 8 to12 kJ, and joint with the heat input of 8 kJ shows the best performance.