When complex structure parts are formed on machined matrix parts by selective laser melting (SLM) technology, large residual stress is easily generated at the interface. The single-layer multi-channel three-dimensional transient numerical model of composite forming 316L stainless steel based on SLM process was established by using ANSYS (APDL) and compared with the experimental results. Based on the finite element model of composite forming 316L stainless steel, the distribution and evolution of temperature field and stress field during composite forming were studied, and the effects of preheating temperature on the thermal behavior, stress evolution, and residual stress distribution of molten pool were analyzed. The results show that the maximum cooling rate, the boundary temperature gradient and the residual stress decrease with the increase of preheating temperature.