Abstract: The keyhole evolution, thermodynamic behavior and surface morphology of 316L molten pool during SLM at mesoscopic scale were analyzed by finite element method.And it was verified by test.It is found that the free surface of the keyhole molten pool is covered by the high-temperature gas of evaporated metal, resulting in the generation of recoil pressure and the deformation of the free surface.In the initial stage of the forming process, the inward convection of the metal at edge of molten pool prevents the expansion of the molten pool, while the subsequent outward convection promotes the melt flow and ensures the overlap of adjacent molten pools after solidification.The recoil pressure towards the surface due to metal vaporization causes a turbulent splashing in the molten pool.These sputters are moved to the outside of the laser path and solidified into a bonded hump structure that could not melt during the formation of next layer.As the number of layers increases, the free surface of the melt pool has concave profiles and material bonding.The local elastic-plastic mechanical properties of three positions of the molten pool were studied by nano indentation experiment.It is found that the indentation hardness and elastic modulus of the lap area in the molten pool are the highest, which are 4.08 GPa and 211.45 GPa, respectively.