In order to solve the quality problem of pore defects in selective laser melting copper alloy parts, the work aims to investigate the kinetic behavior of the multi-track melt pool and the irregular un-fusion pore defect formation mechanism at hatch spacing. During the selective laser melting, the interaction between the material and the laser takes short time and small space scale and is presented throughout the process of heating, melting, flowing and solidification of the powder. The melt pool evolution mechanism can not be observed by experimental means. In order to gain a deep insight into the melt pool dynamics, a high-fidelity numerical model with multi-physics fields is proposed. To simulate the complex process of metal powder melting and even evaporation, multiple phenomena have been considered, such as thermo-capillary forces, Marangoni stress, recoil pressure, temperature-dependent convective and radiative heat flux, and laser-powder interaction. The random packing of the metal powder laid on the substrate is modeled by the commercial DEM software EDEM. The metal phase free surface was traced by the volume of fluid method. The global absorptivity at multi-track forming was also investigated by ray-tracing algorithm. Base on the model, the effect of hatch spacing on melt pool evolution and un-fusion pore defects was analyzed in depth. The results showed that the scan of the previous track had a preheating effect on the latter track, so the width and absorptivity of the second track were always greater than those of the first track. As the scan spacing increased, the heat accumulation effect decreased, so the width and depth of the second melt pool decreased slightly. The global absorptivity curve for each single track could be divided into three processes of increasing stage, decreasing stage, stable stage. The global absorptivity curve of the second track did not vary much with the increasing hatch spacing. A small hatch spacing would increase the overlap between fusion tracks and generate good connection of adjacent tracks without gaps, which was not conducive to improving the building rate of the part. However, an infinite increase in hatch spacing would decrease the overlap zone and even non-connection in the adjacent tracks led to un-fusion pore defects. By comparing the formed tracks at 70 μm, 100 μm and 130 μm hatch spacing, it was found that the overlap rate at 70 μm was close to 50%, 100 μm could form a good overlapping and no defects were observed, while at 130 μm track, obvious pore defects and incompletely melted powder were observed. The different hatch spacing causes changes in liquid melt flow, heat and mass transfer, which in turn changes the melt pool kinetic behavior and absorptivity for multi-track formation of selective laser melting copper alloys, ultimately affecting the quality and mechanical properties of the formed part. This indicates that proper hatch spacing can effectively avoid un-fusion pore defects and ensure a good overlapping between adjacent fusion tracks. The results of this study provide a reference for understanding the melt pool dynamics during selective laser melting of copper alloys and the control of internal pore defects.