Abstract: It takes too long time to repair failed mold with the traditional welding processes because the heat input is dispersed and the property of the heat affected zone is easily deteriorated. The laser welding was used to repair failed mold, which has the advantages of high degree of automation, short work time and concentrated heat input. But the cost of the equipment increases dramatically as the power increases. It can effectively reduce costs that active laser welding technology is applied to mold repairing because weld seam can get a higher depth of weld in low power. The effects of active laser welding on the microstructure and weld appearance at low power were studied by using H13 hot die steel as the experimental subject. The results show that the weld appearance of with the addition of active flux is better, the color of weld is bright and the scaly figure is clear. The active laser welding makes weld width increased by 8%, and weld penetration increased by 66%. No matter coated active flux or not, there are no cracks, porosity, inclusions or other defects in the welds. However, active laser welding can increase the size of the grain in the welds.