The laser fuse additive manufacturing parameters have an important influence on the solidification structure,especially the grain size,which affects the properties of the formed parts. Based on the finite element method,the thermal history of 316L stainless steel was simulated,and the effects of different additive manufacturing processes on grain size were studied. The results show that that the size of the molten pool obtained by simulation is consistent with the reality,and the energy transfer is vertical to the scanning direction and the thickness direction. With the increase of scanning speed,the maximum temperature of forming process decreases. The bottom region of the monolayer microstructure is cellular crystal,the middle region is large columnar dendritic crystal,and the top region is fine equiaxed crystal. The primary dendrite spacing of columnar dendrites reduces as the scanning speed increases.