The forcing parameter is an important value for determining the solidification conditions under electromagnetic impact. This parameter tends to change and affect the solidification conditions. The aim of this work is to investigate the influence of the liquid–solid interface position and deformation on the forcing parameter. Electromagnetic analysis by the finite element method, validated experimentally, was used as the main research tool. Horizontal solidification in the presence of a traveling magnetic field installation was chosen as the experimental setup. Various interface deformations were investigated, as well as its propagation. As a result of the calculation, the electromagnetic (EM) forcing parameter functions were obtained. It was determined that the liquid–solid interface propagation causes significant attenuation of the forcing parameter, as well as interface slope. The results were analyzed and approximated by analytical functions. These functions can be easily used to determine values for various solidification conditions and stages.