In order to clarify the effect of ultrafine spinel sources on slag resistance and microstructure of alumina-spinel castables, using tabular alumina and spinel as coarse aggregates, tabular alumina powders, spinel powders, calcium aluminate cement, spinel containing cement, spinel micro-powder and reactive alumina ultrafines as matrix, alumina-spinel castables used for RH snorkels were prepared by changing the particle size and sources of preformed spinel. The effects of different ultra-fine spinel sources (CMA72, spinel micropowder) on the properties of alumina-spinel castables were investigated. Thermal shock resistance and slag resistance of specimens were compared by the residual strength rate (1 100 ℃, air cooling 1 cycle) and static crucible method. The microstructure and the slag corrosion resistance mechanism of the specimens were analyzed by SEM and EDS. The result shows that slag resistance of alumina-spinel castables can be improved by adding ultra-fine preformed spinel, such as CMA72, and spinel micro-powder. The introduction of ultra-fine preformed spinel has little effect on HMOR of alumina-spinel castables, but has negative effect on TSR. Ultra-fine and dispersive preformed spinel in the matrix and fine microstructure are the main reasons for the improvement of slag resistance of castables.