Abstract: Ti6Al4V alloy was prepared by selective laser melting technique. The effects of laser power, scanning hatch and powder thickness on the compactness of the alloy were studied by orthogonal experiment method, and the formation mechanism of hole defects was discussed. The results show that the powder thickness had the greatest influence on the relative density of the formed alloy, followed by the scanning hatch, and the laser power had the least influence. The relative density of the alloy increased first and then tended to be stable with increasing laser power, decreased with increasing scanning hatch and powder thickness, and increased with increasing bulk energy density. The optimal process was laser power of 200 W, scanning hatch of 0.10 mm and powder thickness of 0.03 mm, at this time the relative density of the alloy was 99.24%. The alloy existed two kinds of hole, including pores and keyholes; the former was mainly caused by the inert gas inside the equipment and powder vapour not escaping in time due to the high temperature gradient inside the molten pool; and the latter was mainly caused by spheroidize and shrink effect of molten pool due to improper process parameters.