Abstract: During the EBW process, the driving force such as recoil pressure induced by evaporation, surface tension and gravity affect the fluid flow and thus influence the bead shape. In the situation of horizontal scanning welding, the scanning oscillation of the heating area of electron beam and the alteration of gravity make the fluid flow in the molten pool more complicated to describe. Experimental and computational methods are used to observe and investigate the shape of the molten pool and the fusion zone after solidification. The penetration depth, melting width and the fusion line profile are predicted accurately by the simulation results compared to the experiments. Analysis of the flow field of molten pool indicates the main driving force of fluid flow in half-penetration weld pool is the recoil pressure. By contrast, the Marangoni flow dominates the fluid flow on the top surface of the full-penetration weld pool, and the surface tension and recoil pressure have a combined action to the molten pool. Gravity and the scanning of electron beam cause the asymmetry of molten pool both in the mass distribution and flow field distribution, and thus the asymmetry of the fusion line profile.