To clarify the control mechanism of scanning spacing on forming quality in selective laser melting (SLM), multiphysics numerical simulation combined with experiments is performed to investigate the mechanism underlying the effect of scanning spacing on the thermodynamic behavior of adjacent melting overlap process. Results show that the preheating effect of the previous scan channel can increase the base temperature and peak temperature of the subsequent scan channel, although this effect weakens as the scanning spacing increases. When the scanning spacing is extremely small (≤50 μm), the temperature of the first zone reaches 1168 K and remelting occurs during the second scanning channel, whereas a larger scanning spacing (≥90 μm) increases the temperature gradient of the fuse and reduces the cooling rate of the scan channel. During weld bonding, the flow rate of the molten pool near the sintered side is sufficient and the melt velocity distribution is uniform, whereas the opposite is observed for the molten pool near the unsintered side. When the scanning spacing is 70 μm, the melt flow velocity difference between the two sides of the molten pool is the smallest, which is more conducive to the stability of the molten pool and the construction quality of SLM, and the relative density and Vickers hardness are 98% and 131.3 HV, respectively. The sweep-spacing parameter should be coordinated with other process parameters to improve the forming quality while ensuring the overlap rate.