Abstract: With IN718 alloy as the research object, a three-dimensional multiphase flow numerical model was established by considering molten pool mass, heat source dissipation, momentum source terms, and solid-liquid phase transformation. The established model was validated through single-layer three-channel laser melting deposition tests. The established model was used to simulate the heat and mass transfer behavior in the molten pool. The remelting phenomena and effects of temperature gradient distribution in the overlapping area on the microstructure were analyzed in combination with test results. The results show that relative errors between the maximum height and width of the single-layer three-channel deposition layer by simulation and test results were less than 6%, and the morphology of deposition layer by simulation was in good agreement with test results, confirming the validity of the established model. The highest temperature of the molten pool first increased, then decreased, and increased during the three-channel laser melting. On the cross section of the molten pool, the molten metal flowed from the center toward both sides. The flow field on the cross section of the first channel molten pool was symmetric about the central axis of the molten pool, and the flow fields of the second and third channel molten pools were inclined toward the overlapping direction. On the longitudinal section, a clockwise vortex was formed at the front of the molten pool, and a counterclockwise vortex was formed at the middle and rear sections. In the second and third channel molten pools, the vortex areas at the middle and rear sections were expanded, and new vortex zones were formed at the rear. Due to remelting and the concentrated asymmetric temperature gradient in the overlapping area, a fine columnar crystal band was formed, which grew along the maximum heat flow direction （toward the subsequent deposition location）.