An electroslag remelting slag system of cobalt-based high temperature alloy comprises, by weight, 65-75% of CaF2, 15-25% of Al2O3, 5-10% of MgO, 2-8% of La2O3, and below 1.5% of impurities. The cobalt-based high temperature alloy prepared by using slag system electroslag remelting has the advantages of low oxygen content, few nonmetal inclusions, great reduction of the burning loss rate of rare earth element La, uniform components, accurate content of the rare earth element, and effectively guaranteed high-temperature oxidation resistance and durability.