Researched on the low carbon steel by mineral component ore phase structure of mould powder and its slag film under optical microscope. The result shows that the glass phase, quartz, and pyroxene in the mould powder helped to the slag film turn into glass. The melilite and wollastonite in the mould powder can promote the formation of melilite, and wollastonite can promote the formation of cuspidine in the slag film. The structure of the slag film is even and the main structure is glass phrase, the rate of crystallization is low, the crystallized mineralogical phases was melilite, the cuspidine did not appeared, which is good for lubrication. The content of glass phase, quartz and pyroxene in the mould powder of the low carbon steel should be increased, the content of wollastonite and fluorite should be reduced.