Inner conductor in Niobium superconducting cavity requires a smooth outer surface after material processing, in order to enable the special performance. In the case of downward EBW with full penetration, defects such as underfill and collapse in the face of weld can happen affected by the recoil pressure. In this paper, proper horizontal EBW parameters are used to achieve weld with front reinforcement in accordance with the technical requirements. A 2-D model for the horizontal welding process of 3mm Niobium plate is constructed, and molten pool models with different size and shape are built for different welding parameters. VOF method is applied to simulate the free-surface status of molten pool metal during the welding process, and to obtain the molten pool evolution for different welding parameters. Experiment result indicates the weld width increases with the rise of heat input, and the simulation result matches the experiment.