The work aims to improve the elongation of Ti6Al4V alloy formed by selective laser melting (SLM), so that the tensile properties of the formed structure can approach or meet the forging standard. The in-situ decomposition method was used to decompose the acicular α‘ martensite in the formed layer into α+β phase by adjusting the process parameters such as laser power, energy density, thickness of the layer and the area ratio of the support in the SLM forming process under the thermal cycle of heat source reciprocating motion for reciprocating heating and cooling. By means of microstructure observations (SEM), phase analysis (XRD) and tensile test, the conditions for in-situ decomposition of Ti6Al4V alloy were determined. Increasing the thickness of SLM layer (60 μm) and laser power (375 W) was beneficial to reducing the cooling rate and temperature gradient in SLM manufacturing process, resulting in a′→a+b transformation of martensite structure. The results of SEM and XRD showed that the morphology of Ti6Al4V alloy after in-situ decomposition was composed of acicular α phase and granular β phase, which was different from that of conventional SLM at high temperature gradient and extremely fast cooling rate. The tensile test results showed that the elongation of Ti6Al4V specimens increased while high yield strength was maintained after in-situ decomposition. The yield strength reached more than 1100 MPa, and the elongation reached 8%. The fracture mechanism was ductile fracture. SLM-formed Ti6Al4V alloy has better toughness after in-situ decomposition, which can improve the tensile properties of Ti6Al4V alloy.