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Because of the addition of significant quantities of Al and Mn, the great challenge of casting 20Mn23Al non-magnetic steel is to design a mold slag that is chemically compatible with this steel chemistry. In this paper, a new mold slag with low melting point, low basicity was proposed. The initial slag (the base mold slag) and two corresponding spent mold slag (8min slag and 15min slag), which was sampled from the caster after 8 minutes and 15minutes of casting, were investigated, the changes of mold slag composition and properties of molten slag were discussed. The results showed that:(i) The slag quickly changed in chemistry to a slag that contained a low quantity of silica and a higher quantity of alumina, and the basicity increased obviously; (ii) Heat flux density through the slag film decreased appreciably with the time increase; (iii) Mold slag used for non-magnetic steel precipitated CaF2 crystals; (iv) The slab has good surface quality with not occurrences of longitudinal face cracks and slag inclusions; (v) The Al2O3 content in the molten slag pool increased with time, and Al2O3 concentration of the pool can be regarded as being in a practically equilibrium state after about 600sec.