Abstract: In this paper, the temperature field of single-layer and multi-beads Selective Laser Melting (SLM) of H13 die steel was simulated by using Gauss body heat source. The heat accumulation of different scanning lengths and its effect on the morphology of molten pool during S-type scanning were studied. The correctness of the simulation was verified by experiments and an improved measure was proposed to reduce the heat accumulation. The results show that the shorter the scanning length, the more serious of the heat accumulation, and accordingly the higher the maximum temperature of the molten pool in the fifth bead. By taking the average growth rates of melting length, melting depth and melting width under different scanning lengths as the standard, the average growth rates of them were 32.1%, 27.1% and 13.5%, respectively. The maximum temperature of the fifth bead was reduced from 3 115.6℃ to 2 881.51℃ when employed a unidirectional scanning at 6 mm. However, the time of completing the same pass is twice as much as that of the S-type scanning.