Experimental investigation and kinetics model ware carried out to study the effect of the atmosphere on the desulfurization of low-sulfur plastic die steel during the electroslag remelting process. 55Cr17Mo1VN plastic die steel was applied as the electrode and remelted with two different kinds of atmospheres using a laboratory-scale ESR furnace. It was found that the sulfur content of 50 ppm in the electrode decreased to 8–12 ppm in the air atmosphere, while reduced to 9–14 ppm in a protective atmosphere. The desulfurization rates were 82% and 78%, respectively. Correspondingly, the sulfur content of 0.12% in initial slag increased to 0.125% and 0.15%. The coupled desulfurization kinetics model was established, the oxygen activity (a_O) and sulfur distribution coefficient (L_s) are taken into consideration, and they change with the remelting time during the calculation. The results show that the calculated values are in good agreement with the experimental values. The desulfurization effect at the electrode tip is significantly better than the positions where the droplet passes through the slag layer and the slag pool/molten pool interface. The L_s and comprehensive mass transfer coefficient of sulfur (k_S^*) decrease with the remelting time, while the a_O at each reaction position increases. Compared with the protective atmosphere, L_s and k_S^* have larger values during the air atmosphere ESR process, but the a_O value is equal. Under the different atmospheres, the most types of inclusions in the steel are MnS, and the refining atmosphere has no significant effect on the types of inclusions.