Since the deformation region involves the interaction of electric-thermal-force coupling in electric hot incremental sheet forming, the numerical simulation of the forming process is unusually difficult. Currently, the thermal-force coupling method is adopted to simulate approximately the whole forming process, and the Joule heating effect is often ignored. Therefore, the numerical simulation of Joule heating effect is especially significant for the prediction accuracy of forming process. In this paper, a novel numerical simulation method, considering electric-thermal-force parameters, was proposed to instantly update the thermal-force condition of forming region. Meanwhile, the model of contact thermal conductance was established combining geometrical and electric-thermal parameters, and then a high-precision finite element model was obtained to predict the Joule heating effect of forming region. In addition to this, the effect of thermal superposition on forming temperature was further analyzed and a modified model of contact thermal conductance was established in electric hot incremental sheet forming.