The heat sources arrangement is very significant for the laser-arc synergetic effects of hybrid welding, but there have been few researches about this. So a detail study of laser-arc hybrid welding process was carried out on the 7 mm mild steel by together a 5 kW CO2 laser with a MAG (metal active gas) welder. The results show that the heat sources arrangement parameters, such as heat sources distance, laser defocused distance, angle of weld torch, etc, have obviously effect on the weld shape, and the good weld shape was obtained only with the appropriate parameter combination. The increment of weld penetration depth caused by heat sources distance is the most, which is 55%, and the effects of wire stick-out and laser defocused distance on the weld depth are also stronger. The weld width is more sensitive to the wire stick-out and the angle of weld torch. The effects of these parameters are achieved by the interaction of three mechanisms: the change of laser-arc plasma interaction, the add state of the two heat sources and the state of the forces on the weld pool.