Abstract: Repairing of casting defect for iron casting has great economic significance.The relationship between welding parameters and joint microstructure along with hardness is investigated by micro-alloyed gray cast iron homogenous electrodes and DC welding machine.The results show that the welding parameters have significant effects on joint microstructure and hardness.Keeping the welding current constant, the higher the preheating temperature is, the more uniform the temperature distribution is and slower the cooling rate of joint is, the much easier gray cast iron microstructure forms.Keeping the preheat temperature constant, with the increase of welding current, welding heat input increases and the cooling rate of joint decreases, which is easy to form gray cast iron microstructure with non-chilled phase.The welding parameters should be controlled in the upper right zone of welding current-preheat temperature-microstructure type diagram.By strictly controlling of welding current and preheat temperature, and using small current to make a rendering combined with high-current continuous welding process, the microstructure and hardness of joint can be effectively controlled, obtaining the homogenous weld with excellent machinability.