Abstract: Titanium alloy plates were welded by laser under vertical positions with the optimized parameters obtained by flat welding positions. For vertical up (VU) welding, burn through holes occurred while the porosity was low. For vertical down (VD) welding, burn through holes didn’t occur while the porosity was high. With volume of fraction (VOF) method, a numerical model was established to track the interface of keyhole, which helped to understand the effect of welding positions on burn through holes and porosity. During VU welding, the welding direction was opposite to gravity, and the hydrostatic force on keyhole was small. The keyhole was easy to be expanded. When the liquid metal could not fill in the cavity of keyhole in time, burn through holes occurred. During VD welding, the welding direction was the same as gravity. The oscillation of keyhole size resulted to the collapse of keyhole and the entrapment of gas. Since there was lack of escape ways for entrapped gas, porosity increased during VD welding.