In this study, the 3D morphology of a keyhole after droplets fill the molten pool and the metal flow behavior of the molten pool during the process of droplets of different size filling the molten pool by corresponding numerical simulation are studied. Numerical simulation results show that the droplets above and below the keyhole that fall into the molten pool have a greater influence on the morphology fluctuation of the keyhole. Larger droplets increase the keyhole depth fluctuation. After a droplet with a radius of 0.9 mm enters the molten pool, the tendency of the liquid metal on the front wall of the keyhole to flow from the top to the bottom of the molten pool is increased, and a downward flow trend is observed near the bottom of the keyhole. The flow trend before a droplet enters the molten pool is diametrically opposite. After a droplet with a radius of 0.6 mm enters the molten pool, the liquid metal flow trend in the front wall of the keyhole is more complicated, the upper part of the front wall of the keyhole has a tendency to flow from the surface to the bottom of the molten pool. The flow trend from left to right in the middle of the front wall of the keyhole and a trend from the bottom of the molten pool to the top of the bottom of the keyhole occurs。