Abstract: The physical and chemical properties, and structure of crystallizing phases of two kinds of flux • 56 ~ 58 melilite, 25 -27 grammite, 4 ~5 lance spar, 10 - 12 glass phase and 20 -22 melilite, 78 ~80 glass phase, and ingredient, melting temperature and thickness of liquid slag layer used for 180 - 200 mm slab casting at Tangshan stainless steel mill have been tested and studied. Results showed that as casting speed >1.5 m/min, in order to make the flux maintains " whole course liquid lubrication" in mold, the melting temperature of flux should be W1 049 乜 with controlling melting speed (34 ±2) s； and the optimum thickness of liquid slag layer is 10 ~ 15 mm with slag viscosity 0. 19 Pa • s.