The invention discloses an electroslag remelting slag system for duplex stainless steel. The electroslag remelting slag system is prepared from the following components in percent by weight: 50.0-65.0% of CaF2, 8.0-15.0% of Al2O3, 3.0-10.0% of CeO2, 15.0-20.0% of CaO, 3.0-5.0% of MgO, 1.0-3.0% of SiO2 and the balance impurities which are not more than 1%. According to the slag system, the slag viscosity changes gently as the temperature is reduced, and the high temperature plasticity and the strength of slag can be also improved, so that bleed-out and slag leakage are prevented. The conductivity of the slag can be reduced, the chemical stability of the slag can be improved, and the system has better deoxidizing and desulfurizing effects and inclusion absorption action, thereby playing a role of purifying molten steel deeply. As-cast grains can be refined, and the strength and the toughness of steel are improved. The electroslag remelting slag system is uniform to transfer heat and goodin lubricating effect, and the slag layer is kept stable and is melted uniformly.