Experimental studies have revealed that during electromagnetic launches, the molten deposits on the rail surface exhibits contraction behavior. This indicates that a specific force at the rail-armature interface dominates the flow behavior of the molten metal film. The most probable candidate for this force is electromagnetic force, which directly depends on the spatial distribution of current density. Therefore, this study develops a 3-D electromagnetic finite element simulation model of the railgun system to investigate the current density distribution and electromagnetic force characteristics within the rail-armature system. Simulation results indicate that under pulsed current excitation, the electromagnetic force initially serves as an inward-directed constraining force during the current-rising stage, maintaining a stable distribution of the molten metal film at interface. However, in the current-falling stage, due to the rapid reduction and eventual reversal of the current, electromagnetic force transitions from an inward-constraining force into an outward-expanding force. This shift drives the molten metal film outward, causing a loss of material at the interface and ultimately resulting in the observed contraction phenomenon of the molten deposits.