Taking the cooling rate as the cut-in point, the effects of cooling rates, power densities and interaction time on the hardened layer of ductile cast iron were studied in the process of laser heat treatment. Through theoretical calculations, different cooling rate distributions were obtained by using different power densities and interaction times, and laser surface heat treatment was performed on QT600-3 ductile iron. After laser heat treatment , it is found that the surface temperature of each group of parameters is 1 500~2 500 ℃, and the actual cooling rate is about 6% compared with the theoretical calculation above. The melting zone can occur in each group of hardened layers, but the depth, hardness and grain distribution are different. The depth variation of each group is 0.5~0.8 mm, the micro hardness varies from 800 to 1100 HV0.1, and the grains are coarse to fine. The microstructure and hardness of the fused zone are most affected by the cooling rate. Due to the large distance from the surface in other regions, the cooling rates of each group are greatly tend to be consistent, so the hardness change is not obvious.The depth of the hardened layer is proportional to the cooling rate, and gradually increases with the increase of the cooling rate. Under the premise of ensuring the same laser energy density, the power density has a greater influence on the depth than the interaction time.