Selective laser melting (SLM) is used to prepare 316L stainless steels. Further, the effects of laser power, scanning speed, and scanning spacing on the formation of cracks are studied in detail, the morphologies, chemical compositions, types of the precipitated phases, and grain sizes of the cracks are discussed, and the microstructures and formation mechanism of the cracks at different positions are presented. The results denote that the cracks mainly include micropore aggregation cracks, bubble aggregation cracks, and hot cracks. With the increase of linear energy density, the numbers of micropore aggregation cracks and bubble aggregation cracks are observed to initial increase and subsequent decrease; however, the number of hot cracks is observed to unidirectional increase. Under the optimal process parameters (a linear energy density of 222.2 J/m, a laser power of 200 W, and a laser scanning speed of 900 mm/s), the samples containing a small number of pores but no lacking cracks and no bubbles are obtained.