A transient three-dimensional (3D) model was developed to understand the role slag thickness plays in the formation of the metal pool in the electroslag remelting (ESR) process. In this model, the solution of the mass, momentum and energy conservation equations were simultaneously implemented by the finite volume method with full coupling of the Joule heating and Lorentz force by solving the Maxwell’s equations. The movement of metal droplet was described by volume of fluid (VOF) approach. Additionally, the solidification was modeled using an enthalpy-based technique, where the mushy zone was treated as a porous medium. The experiment and simulation demonstrated a reasonable agreement. The results indicate that changing the slag thickness changes the slag temperature, but not monotonically. The slag temperature drops with the slag thickness up to 60 mm, beyond which the slag temperature rises. The melt rate decreases and then increases while the cooling intensity remains unchanged. As a consequence, the maximal metal pool depth reduces from 0.081 m to 0.067 m and slightly increases to 0.074 m.