Using temary-quadratic orthogonal design method the viscosities of CaO-MgO-SiO

2

-Al

2

O

3

quaternary refining slag series with basicity (CaO/SiO

2

) 1.2-6.0

MgO 1.9%-6.0% and Al

2

O

3

6%-29.1% are measured and studied by using comprehensive meter for melt properties. Test results show that the viscosities of quaternary slag series at 1500 ℃ are 0.129-4.612 Pa·s； with increasing basicity the viscosity of refining slag series decreases slightly at beginning then increases quickly ； with increasing MgO content the viscosity of slag series decreases remarkably at first then increases slightly and with increasing Al

2

O

3

content the viscosity of slag series decreases rapidly. It is obtained by synthetical study that the refining slag series with basicity 4-5

MgO 4.5%-5.5% and Al

2

O

3

24%-27% have suitable viscosity (≤ 1.0 Pa

·

s) which have better fluidity to promote slag-metal reaction

increase ability of absorption of inclusions and create a favorable condition for desulphurization.