The quality control problem of the stitching overlap area of multi-beam selective lasermelting processing was studied to investigate the defect formation mechanism and control means of the stitching overlap area. Due to the difficult observation of the interaction behavior between laser and material during selective laser melting, based on the volume of fluid method and ray-traced heat source, the process of powder melting and solidification was restored by establishing a high-fidelity numerical model of selective laser melting at mesoscopic scale to study the melt pool dynamics and laser reflection & absorption behaviors in the spliced overlap region under different processing parameters, and to compare and analyze the laser-material energy coupling mechanism in the spliced overlap region and the non-spliced overlap region, to explore means of suppressing defects in overlapping regions of splicing. The numerical simulation model was based on computational fluid dynamics theory and a multiphase flow model with the finite volume method was used and combined with physical field models such as melting and solidification, heat loss, surface tension, recoil pressure, and ray tracing heat source. A single-pass numerical simulation of laser-selected melting was performed at a laser power of 430 W, in which there was no interruption time between the two laser beams, and the second beam was processed immediately after the completion of the first beam, and the two beam scans were of equal length. The two beams were in one scanning path, and the starting point of the second beam scan was before the end point of the first scan, resulting in a stitching overlap region. Numerical simulations were performed at the same stitching overlap area scan speed, different stitching overlap area size and the same stitching overlap area size, different stitching overlap area scan speed, where the non-stitching overlap area scan speed in both cases was 0.6 m/s. With different sizes of spliced overlap regions, the spliced overlap regions had wider fusion channel width than the non-spliced overlap regions when the length of the overlap regions was 160 μm, 200 μm and 240 μm, respectively. There was a height difference between the spliced overlap regions and the non-spliced overlap regions, and the global laser absorption rate of the overlap regions was higher than that of the non-overlap regions, where the overlap regions all had porosity defects, and the highest global average absorption rate was 0.417 56 for the 240 μm overlap region. The highest global average absorption rate was 0.417 56 for the 240 μm length of the overlapped area. The energy obtained was lower than that obtained at the scanning speed of 0.9 m/s and 1.2 m/s for the 240 μm length of the overlapped area. There were no porosity defects in the overlapped area. By analyzing the results of numerical simulations and experimental situations studied by other scholars, the surface morphology and porosity defects of the splice overlap region are closely related to the melt pool dynamics and laser reflection and absorption behaviors. Suitable processing parameters can improve the forming quality of the splice overlap region. This research can provide a reference for multi-beam laser selective melting and splicing overlap area forming.