The influence of welding parameters on porosity in the welding of TC4 titanium alloy in flat and horizontal positions was investigated. Results showed that parameters had different influences on porosity in different positions. In flat position, the amount of porosity decreased first and then increased with the increase of laser power and defocus, it increased first and then decreased with the increase of welding speed; in horizontal position, the amount of porosity decreased with the decrease of laser power and the increase of welding speed, and it decreased first and then increased with the increase of defocus. By optimization of parameters with the range analysis method, the amount of porosity is reduced significantly in weld beads, and welds with good inner quality could be produced under both flat and horizontal welding positions.