Abstract: The microstructures of accidental film and adjusted film for low-carbon steel have been studied,accidental reasons have been analysed,and the way is point out to improve performance of lubricity and heat conductivity of casting powder and quality of slab by controlling the solidified film structure and mineralogical phase.Casting powder for low-carbon steel should possess good heat conductivity and low crystal rate.so,It is necessary to decrease crystal rate and increase glassy tendency of film,and avoid forming some mineral like K-Na-feldspar、Plagioclase、nephelite、cuspidine in solidfied film.All things considered,especially lubricity and heat conductivity,wollastonite、melilite in solidfied film are good.After adding B₂O₃ and LiO₂,crystal size was decreased,glassy tendency was increased,and the quality of slab was improved obviously.