This paper introduces the requirement of mold casting on protection slag and the function of commonly used additives， and according to the character such as composition， melting point of M2 high speed steel， fluorite powder， alkali powder and expandable graphite are added to the universal mold casting protective slag， and the melting point， viscosity and spread of the protective slag are adjusted， and a lot of tests have been carried out in the production . When fluorite powder is added more than 7% alone， or fluorite powder and alkali powder are added more than 4% at the same time， the proportion of slag area on the surface of M2 high speed steel mold ingot is reduced to 2%， but the ingot body deteriorates seriously， more than 5% of the pits appear. When fluorite powder， alkali powder and expandable graphite are added 4%， the proportion of slag area on the ingot surface is reduced to 0%， and the surface of the ingot body is good， which meets the requirements of M2 Die casting high-speed steel . Suitable for M2 series of die casting high-speed steel protective slag， its chemical composition is （18-21） %TC，（29-34） % SiO

2

，（14-17）% Al

2

O

3

，（12-15）% CaO，（4-7）% Na

2

O，（2-4）% Fe

2

O

3

，（1-3）% MgO，（2-4）%F，（0.5-2）% K

2

O.