This study systematically investigated the effects of groove type and welding parameters on droplet transfer, arc shape, and weld formation during laser-MIG hybrid backing welding of 20 mm thick 6005A-T6 aluminum alloy for rail trains. The results indicated that U-shaped groove with root face height of 10 mm was more conducive to obtaining stable welding process. The arc shape was stable and no spatters when the arc current was 280 A, the main types of droplet transition were spray transfer and jet transfer. The arc was easily attracted by the side wall and welding spatter was large when the arc current was low. The stability of the arc reduced and the droplet affected the weld formation by spherical melt transition. The droplet transition frequency increased slightly as the welding speed increased, and the welding fish scales became more obvious. The laser could compress and stabilize the arc form when the heat resource distance was about 3 mm, and the coupling effect of the heat source reduced if the distance between heat sources was too large or too small.