•Rapid softening and hardening phenomenon are found in the SLM Ti6Al4V alloy after EPT.•The changes of microhardness were attributed to the phase transformation and microstructure evolution.•EPT induced rapid α → β phase transformation by decreasing transformation barrier in the SLM Ti6Al4V alloy.•The improvement of metallurgical bonding of the α lamella after EPT may enhance the mechanical properties. Display omitted Optimizing the microstructure and mechanical properties of the selective laser melted (SLM) Ti6Al4V alloy is an important issue both in the scientific and engineering interests. In this study, the rapid hardening and softening of the SLM-Ti6Al4V alloy is realized due to the obvious evolution of microstructures during the electropulsing treatment (EPT) with different discharge voltages. The microhardness of the EPT-7.5 kV sample decreased about 7% while the EPT-8 kV sample increased about 10% compared with the as-received sample. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analysis were used to characterize the evolutions of α and β grains and dislocation structures which are corresponding with the microhardness changes induced by EPT. Analysis shows that EPT could reduce the thermodynamic barrier for α → β solid-state phase transformation, further arise microstructure refinement in the SLM-Ti6Al4V alloy. This research shed some new light on rapidly and efficiently improving the microstructure and mechanical properties of the additive manufactured metal components.