To further investigate the porosity evolution during selective laser melting (SLM) Inconel 718 alloy, a transient mesoscale model with a randomly packed powder-bed has been proposed by finite volume method (FVM), taking consideration of the phase transition, variation of thermo-physical properties and interfacial force. The thermodynamics within molten pool and resulting porosity evolution behavior of a set of laser scanned tracks with various laser scanning speeds were studied using numerical approach. The results evidently revealed that the operating peak temperature was reduced obviously as increasing the scanning speeds. Accordingly, the high cooling rate, short lifespan and limiting depth of pool and small velocity of molten liquid flow were obtained under a high scanning speed. Scanning speed played a crucial role in determining the type of porosity in the terminally SLM-processed Inconel 718 components. At a high scanning speed of 500 mm/s, the top surface was primarily dominated by open porosity, accompanying with large-sized inter-layer porosity on the cross section, due to a limiting energy input penetrated into the powder-bed and incomplete melting of powder. By contrast, as a relatively low scanning speed of 200 mm/s was employed, the top surface appeared to be smooth free of less metallurgical porosity and no apparent inter-layer porosity on the cross section surface attributing to the escaping of porosity, indicating an well metallurgical bonding of the neighboring layer towards the building direction. Simultaneously, the physical mechanism was thoroughly discussed. The simulated distribution of porosity was found to be consistent with the experimental measurements.