Single-phase-to-ground faults in distribution network cables occur frequently and are difficult to detect. Researches on the transient process of these faults commonly rely on arc models. However, existing arc models primarily focus on circuit characteristics and cannot accurately reflect the transient behavior of cable single-phase-to-ground faults. Based on full-scale tests and magnetohydrodynamics (MHD) simulations, this paper investigates the physical evolution of arcs and establishes a field-circuit coupled arc model for single-phase-to-ground faults. First, based on full-scale experiments, MHD simulations of arcs under cable single-phase-to-ground fault conditions are conducted to develop an arc physical field model. Subsequently, key parameters of the arc circuit model—such as arc dissipation power, time constant, and conductance—are obtained from the physical field simulations. The arc dissipation power and time constant are then reconstructed as functions of arc conductance and fault current level, and combined with the circuit model to establish the arc field-circuit coupling relationship. Finally, the relative errors between the simulation results of various arc models and the transient characteristics obtained from full-scale tests are compared and analyzed. Meanwhile, the applicability of the proposed arc field-circuit coupled model and the variations in transient characteristics under different fault current levels are discussed. The results show that the relative errors between the simulation waveforms and the full-scale experimental transient characteristics are all less than 5%, verifying the accuracy of the proposed model.